diff --git a/accessible/base/AccIterator.cpp b/accessible/base/AccIterator.cpp
index dd47b02127..80412cda99 100644
--- a/accessible/base/AccIterator.cpp
+++ b/accessible/base/AccIterator.cpp
@@ -141,7 +141,7 @@ HTMLLabelIterator::Next()
   // element, or <label> ancestor which implicitly point to it.
   Accessible* label = nullptr;
   while ((label = mRelIter.Next())) {
-    if (label->GetContent()->Tag() == nsGkAtoms::label)
+    if (label->GetContent()->IsHTMLElement(nsGkAtoms::label))
       return label;
   }
 
@@ -155,17 +155,15 @@ HTMLLabelIterator::Next()
   Accessible* walkUp = mAcc->Parent();
   while (walkUp && !walkUp->IsDoc()) {
     nsIContent* walkUpElm = walkUp->GetContent();
-    if (walkUpElm->IsHTML()) {
-      if (walkUpElm->Tag() == nsGkAtoms::label &&
-          !walkUpElm->HasAttr(kNameSpaceID_None, nsGkAtoms::_for)) {
-        mLabelFilter = eSkipAncestorLabel; // prevent infinite loop
-        return walkUp;
-      }
-
-      if (walkUpElm->Tag() == nsGkAtoms::form)
-        break;
+    if (walkUpElm->IsHTMLElement(nsGkAtoms::label) &&
+        !walkUpElm->HasAttr(kNameSpaceID_None, nsGkAtoms::_for)) {
+      mLabelFilter = eSkipAncestorLabel; // prevent infinite loop
+      return walkUp;
     }
 
+    if (walkUpElm->IsHTMLElement(nsGkAtoms::form))
+      break;
+
     walkUp = walkUp->Parent();
   }
 
@@ -188,7 +186,7 @@ HTMLOutputIterator::Next()
 {
   Accessible* output = nullptr;
   while ((output = mRelIter.Next())) {
-    if (output->GetContent()->Tag() == nsGkAtoms::output)
+    if (output->GetContent()->IsHTMLElement(nsGkAtoms::output))
       return output;
   }
 
@@ -211,7 +209,7 @@ XULLabelIterator::Next()
 {
   Accessible* label = nullptr;
   while ((label = mRelIter.Next())) {
-    if (label->GetContent()->Tag() == nsGkAtoms::label)
+    if (label->GetContent()->IsXULElement(nsGkAtoms::label))
       return label;
   }
 
@@ -234,7 +232,7 @@ XULDescriptionIterator::Next()
 {
   Accessible* descr = nullptr;
   while ((descr = mRelIter.Next())) {
-    if (descr->GetContent()->Tag() == nsGkAtoms::description)
+    if (descr->GetContent()->IsXULElement(nsGkAtoms::description))
       return descr;
   }
 
diff --git a/accessible/base/Logging.cpp b/accessible/base/Logging.cpp
index 0975307f9b..a080c35e57 100644
--- a/accessible/base/Logging.cpp
+++ b/accessible/base/Logging.cpp
@@ -713,7 +713,7 @@ logging::Node(const char* aDescr, nsINode* aNode)
   dom::Element* elm = aNode->AsElement();
 
   nsAutoCString tag;
-  elm->Tag()->ToUTF8String(tag);
+  elm->NodeInfo()->NameAtom()->ToUTF8String(tag);
 
   nsIAtom* idAtom = elm->GetID();
   nsAutoCString id;
diff --git a/accessible/base/TextAttrs.cpp b/accessible/base/TextAttrs.cpp
index d9daab9568..cf19f2054c 100644
--- a/accessible/base/TextAttrs.cpp
+++ b/accessible/base/TextAttrs.cpp
@@ -872,11 +872,10 @@ TextAttrsMgr::TextPosTextAttr::
   }
 
   const nsIContent* content = aFrame->GetContent();
-  if (content && content->IsHTML()) {
-    const nsIAtom* tagName = content->Tag();
-    if (tagName == nsGkAtoms::sup)
+  if (content) {
+    if (content->IsHTMLElement(nsGkAtoms::sup))
       return eTextPosSuper;
-    if (tagName == nsGkAtoms::sub)
+    if (content->IsHTMLElement(nsGkAtoms::sub))
       return eTextPosSub;
   }
 
diff --git a/accessible/base/nsAccessibilityService.cpp b/accessible/base/nsAccessibilityService.cpp
index 0c1b386b72..e4823e2675 100644
--- a/accessible/base/nsAccessibilityService.cpp
+++ b/accessible/base/nsAccessibilityService.cpp
@@ -889,7 +889,7 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
 
 #ifdef DEBUG
   nsImageFrame* imageFrame = do_QueryFrame(frame);
-  NS_ASSERTION(imageFrame && content->IsHTML() && content->Tag() == nsGkAtoms::area,
+  NS_ASSERTION(imageFrame && content->IsHTMLElement(nsGkAtoms::area),
                "Unknown case of not main content for the frame!");
 #endif
     return nullptr;
@@ -897,7 +897,7 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
 
 #ifdef DEBUG
   nsImageFrame* imageFrame = do_QueryFrame(frame);
-  NS_ASSERTION(!imageFrame || !content->IsHTML() || content->Tag() != nsGkAtoms::area,
+  NS_ASSERTION(!imageFrame || !content->IsHTMLElement(nsGkAtoms::area),
                "Image map manages the area accessible creation!");
 #endif
 
@@ -926,8 +926,7 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
     return newAcc;
   }
 
-  bool isHTML = content->IsHTML();
-  if (isHTML && content->Tag() == nsGkAtoms::map) {
+  if (content->IsHTMLElement(nsGkAtoms::map)) {
     // Create hyper text accessible for HTML map if it is used to group links
     // (see http://www.w3.org/TR/WCAG10-HTML-TECHS/#group-bypass). If the HTML
     // map rect is empty then it is used for links grouping. Otherwise it should
@@ -966,7 +965,7 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
     roleMapEntry = nullptr;
   }
 
-  if (!newAcc && isHTML) {  // HTML accessibles
+  if (!newAcc && content->IsHTMLElement()) {  // HTML accessibles
     bool isARIATableOrCell = roleMapEntry &&
       (roleMapEntry->accTypes & (eTableCell | eTable));
 
@@ -1004,9 +1003,9 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
                  aContext->ARIARoleMap() == &aria::gEmptyRoleMap) {
         roleMapEntry = &aria::gEmptyRoleMap;
 
-      } else if (content->Tag() == nsGkAtoms::dt ||
-                 content->Tag() == nsGkAtoms::li ||
-                 content->Tag() == nsGkAtoms::dd ||
+      } else if (content->IsAnyOfHTMLElements(nsGkAtoms::dt,
+                                              nsGkAtoms::li,
+                                              nsGkAtoms::dd) ||
                  frame->AccessibleType() == eHTMLLiType) {
         nsRoleMapEntry* contextRoleMap = aContext->ARIARoleMap();
         if (!contextRoleMap->IsOfType(eList))
@@ -1016,7 +1015,7 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
   }
 
   // Accessible XBL types and deck stuff are used in XUL only currently.
-  if (!newAcc && content->IsXUL()) {
+  if (!newAcc && content->IsXULElement()) {
     // No accessible for not selected deck panel and its children.
     if (!aContext->IsXULTabpanels()) {
       nsDeckFrame* deckFrame = do_QueryFrame(frame->GetParent());
@@ -1045,18 +1044,18 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
   }
 
   if (!newAcc) {
-    if (content->IsSVG()) {
+    if (content->IsSVGElement()) {
       nsSVGPathGeometryFrame* pathGeometryFrame = do_QueryFrame(frame);
       if (pathGeometryFrame) {
         // A graphic elements: rect, circle, ellipse, line, path, polygon,
         // polyline and image. A 'use' and 'text' graphic elements require
         // special support.
         newAcc = new EnumRoleAccessible(content, document, roles::GRAPHIC);
-      } else if (content->Tag() == nsGkAtoms::svg) {
+      } else if (content->IsSVGElement(nsGkAtoms::svg)) {
         newAcc = new EnumRoleAccessible(content, document, roles::DIAGRAM);
       }
-    } else if (content->IsMathML()){
-      if (content->Tag() == nsGkAtoms::math)
+    } else if (content->IsMathMLElement()) {
+      if (content->IsMathMLElement(nsGkAtoms::math))
         newAcc = new EnumRoleAccessible(content, document, roles::EQUATION);
       else
         newAcc = new HyperTextAccessible(content, document);
@@ -1067,13 +1066,15 @@ nsAccessibilityService::GetOrCreateAccessible(nsINode* aNode,
   // of some property that makes this object interesting
   // We don't do this for <body>, <html>, <window>, <dialog> etc. which
   // correspond to the doc accessible and will be created in any case
-  if (!newAcc && content->Tag() != nsGkAtoms::body && content->GetParent() &&
+  if (!newAcc && !content->IsHTMLElement(nsGkAtoms::body) &&
+      content->GetParent() &&
       (roleMapEntry || MustBeAccessible(content, document) ||
-       (isHTML && nsCoreUtils::HasClickListener(content)))) {
+       (content->IsHTMLElement() &&
+        nsCoreUtils::HasClickListener(content)))) {
     // This content is focusable or has an interesting dynamic content accessibility property.
     // If it's interesting we need it in the accessibility hierarchy so that events or
     // other accessibles can point to it, or so that it can hold a state, etc.
-    if (isHTML) {
+    if (content->IsHTMLElement()) {
       // Interesting HTML container which may have selectable text and/or embedded objects
       newAcc = new HyperTextAccessibleWrap(content, document);
     } else {  // XUL, SVG, MathML etc.
@@ -1253,7 +1254,7 @@ nsAccessibilityService::CreateAccessibleByType(nsIContent* aContent,
 
     for (nsIContent* child = listItem->GetFirstChild(); child;
          child = child->GetNextSibling()) {
-      if (child->IsXUL(nsGkAtoms::listcell) && child != aContent) {
+      if (child->IsXULElement(nsGkAtoms::listcell) && child != aContent) {
         accessible = new XULListCellAccessibleWrap(aContent, aDoc);
         break;
       }
@@ -1285,7 +1286,7 @@ nsAccessibilityService::CreateAccessibleByType(nsIContent* aContent,
     // implementations on each platform for a consistent scripting environment, but
     // then strip out redundant accessibles in the AccessibleWrap class for each platform.
     nsIContent *parent = aContent->GetParent();
-    if (parent && parent->IsXUL() && parent->Tag() == nsGkAtoms::menu)
+    if (parent && parent->IsXULElement(nsGkAtoms::menu))
       return nullptr;
 #endif
 
@@ -1379,45 +1380,45 @@ nsAccessibilityService::CreateHTMLAccessibleByMarkup(nsIFrame* aFrame,
   }
 
   // This method assumes we're in an HTML namespace.
-  nsIAtom* tag = aContent->Tag();
-  if (tag == nsGkAtoms::figcaption) {
+  if (aContent->IsHTMLElement(nsGkAtoms::figcaption)) {
     nsRefPtr<Accessible> accessible =
       new HTMLFigcaptionAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::figure) {
+  if (aContent->IsHTMLElement(nsGkAtoms::figure)) {
     nsRefPtr<Accessible> accessible =
       new HTMLFigureAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::legend) {
+  if (aContent->IsHTMLElement(nsGkAtoms::legend)) {
     nsRefPtr<Accessible> accessible =
       new HTMLLegendAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::option) {
+  if (aContent->IsHTMLElement(nsGkAtoms::option)) {
     nsRefPtr<Accessible> accessible =
       new HTMLSelectOptionAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::optgroup) {
+  if (aContent->IsHTMLElement(nsGkAtoms::optgroup)) {
     nsRefPtr<Accessible> accessible =
       new HTMLSelectOptGroupAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::ul || tag == nsGkAtoms::ol ||
-      tag == nsGkAtoms::dl) {
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::ul,
+                                    nsGkAtoms::ol,
+                                    nsGkAtoms::dl)) {
     nsRefPtr<Accessible> accessible =
       new HTMLListAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::a) {
+  if (aContent->IsHTMLElement(nsGkAtoms::a)) {
     // Only some roles truly enjoy life as HTMLLinkAccessibles, for details
     // see closed bug 494807.
     nsRoleMapEntry* roleMapEntry = aria::GetRoleMap(aContent);
@@ -1438,13 +1439,13 @@ nsAccessibilityService::CreateHTMLAccessibleByMarkup(nsIFrame* aFrame,
     // it unconditionally by tag name. nsBlockFrame creates the list item
     // accessible for other elements styled as list items.
     if (aContext->GetContent() == aContent->GetParent()) {
-      if (tag == nsGkAtoms::dt || tag == nsGkAtoms::li) {
+      if (aContent->IsAnyOfHTMLElements(nsGkAtoms::dt, nsGkAtoms::li)) {
         nsRefPtr<Accessible> accessible =
           new HTMLLIAccessible(aContent, document);
         return accessible.forget();
       }
 
-      if (tag == nsGkAtoms::dd) {
+      if (aContent->IsHTMLElement(nsGkAtoms::dd)) {
         nsRefPtr<Accessible> accessible =
           new HyperTextAccessibleWrap(aContent, document);
         return accessible.forget();
@@ -1454,42 +1455,42 @@ nsAccessibilityService::CreateHTMLAccessibleByMarkup(nsIFrame* aFrame,
     return nullptr;
   }
 
-  if (tag == nsGkAtoms::abbr ||
-      tag == nsGkAtoms::acronym ||
-      tag == nsGkAtoms::article ||
-      tag == nsGkAtoms::aside ||
-      tag == nsGkAtoms::blockquote ||
-      tag == nsGkAtoms::form ||
-      tag == nsGkAtoms::footer ||
-      tag == nsGkAtoms::header ||
-      tag == nsGkAtoms::h1 ||
-      tag == nsGkAtoms::h2 ||
-      tag == nsGkAtoms::h3 ||
-      tag == nsGkAtoms::h4 ||
-      tag == nsGkAtoms::h5 ||
-      tag == nsGkAtoms::h6 ||
-      tag == nsGkAtoms::nav ||
-      tag == nsGkAtoms::q ||
-      tag == nsGkAtoms::section ||
-      tag == nsGkAtoms::time) {
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::abbr,
+                                    nsGkAtoms::acronym,
+                                    nsGkAtoms::article,
+                                    nsGkAtoms::aside,
+                                    nsGkAtoms::blockquote,
+                                    nsGkAtoms::form,
+                                    nsGkAtoms::footer,
+                                    nsGkAtoms::header,
+                                    nsGkAtoms::h1,
+                                    nsGkAtoms::h2,
+                                    nsGkAtoms::h3,
+                                    nsGkAtoms::h4,
+                                    nsGkAtoms::h5,
+                                    nsGkAtoms::h6,
+                                    nsGkAtoms::nav,
+                                    nsGkAtoms::q,
+                                    nsGkAtoms::section,
+                                    nsGkAtoms::time)) {
     nsRefPtr<Accessible> accessible =
       new HyperTextAccessibleWrap(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::label) {
+  if (aContent->IsHTMLElement(nsGkAtoms::label)) {
     nsRefPtr<Accessible> accessible =
       new HTMLLabelAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::output) {
+  if (aContent->IsHTMLElement(nsGkAtoms::output)) {
     nsRefPtr<Accessible> accessible =
       new HTMLOutputAccessible(aContent, document);
     return accessible.forget();
   }
 
-  if (tag == nsGkAtoms::progress) {
+  if (aContent->IsHTMLElement(nsGkAtoms::progress)) {
     nsRefPtr<Accessible> accessible =
       new HTMLProgressMeterAccessible(aContent, document);
     return accessible.forget();
@@ -1607,7 +1608,7 @@ nsAccessibilityService::CreateAccessibleByFrameType(nsIFrame* aFrame,
       newAcc = new HTMLTextFieldAccessible(aContent, document);
       break;
     case eHyperTextType:
-      if (aContent->Tag() != nsGkAtoms::dt && aContent->Tag() != nsGkAtoms::dd)
+      if (!aContent->IsAnyOfHTMLElements(nsGkAtoms::dt, nsGkAtoms::dd))
         newAcc = new HyperTextAccessibleWrap(aContent, document);
       break;
 
diff --git a/accessible/base/nsTextEquivUtils.cpp b/accessible/base/nsTextEquivUtils.cpp
index d2ceab7701..9825a92cfc 100644
--- a/accessible/base/nsTextEquivUtils.cpp
+++ b/accessible/base/nsTextEquivUtils.cpp
@@ -153,7 +153,7 @@ nsTextEquivUtils::AppendTextEquivFromTextContent(nsIContent *aContent,
     return NS_OK;
   }
   
-  if (aContent->IsHTML() &&
+  if (aContent->IsHTMLElement() &&
       aContent->NodeInfo()->Equals(nsGkAtoms::br)) {
     aString->AppendLiteral("\r\n");
     return NS_OK;
@@ -301,7 +301,7 @@ nsTextEquivUtils::AppendFromDOMNode(nsIContent *aContent, nsAString *aString)
   if (rv != NS_OK_NO_NAME_CLAUSE_HANDLED)
     return NS_OK;
 
-  if (aContent->IsXUL()) {
+  if (aContent->IsXULElement()) {
     nsAutoString textEquivalent;
     nsCOMPtr<nsIDOMXULLabeledControlElement> labeledEl =
       do_QueryInterface(aContent);
diff --git a/accessible/generic/Accessible-inl.h b/accessible/generic/Accessible-inl.h
index 460f60c34c..93017f8b4d 100644
--- a/accessible/generic/Accessible-inl.h
+++ b/accessible/generic/Accessible-inl.h
@@ -48,7 +48,7 @@ inline bool
 Accessible::IsSearchbox() const
 {
   return (mRoleMapEntry && mRoleMapEntry->Is(nsGkAtoms::searchbox)) ||
-    (mContent->IsHTML(nsGkAtoms::input) &&
+    (mContent->IsHTMLElement(nsGkAtoms::input) &&
      mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                            nsGkAtoms::textInputType, eCaseMatters));
 }
diff --git a/accessible/generic/Accessible.cpp b/accessible/generic/Accessible.cpp
index dfaa5e113d..9cd70b8e16 100644
--- a/accessible/generic/Accessible.cpp
+++ b/accessible/generic/Accessible.cpp
@@ -141,22 +141,22 @@ Accessible::Name(nsString& aName)
     return nameFlag;
 
   // In the end get the name from tooltip.
-  if (mContent->IsHTML()) {
+  if (mContent->IsHTMLElement()) {
     if (mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::title, aName)) {
       aName.CompressWhitespace();
       return eNameFromTooltip;
     }
-  } else if (mContent->IsXUL()) {
+  } else if (mContent->IsXULElement()) {
     if (mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::tooltiptext, aName)) {
       aName.CompressWhitespace();
       return eNameFromTooltip;
     }
-  } else if (mContent->IsSVG()) {
+  } else if (mContent->IsSVGElement()) {
     // If user agents need to choose among multiple ‘desc’ or ‘title’ elements
     // for processing, the user agent shall choose the first one.
     for (nsIContent* childElm = mContent->GetFirstChild(); childElm;
          childElm = childElm->GetNextSibling()) {
-      if (childElm->IsSVG(nsGkAtoms::desc)) {
+      if (childElm->IsSVGElement(nsGkAtoms::desc)) {
         nsTextEquivUtils::AppendTextEquivFromContent(this, childElm, &aName);
         return eNameFromTooltip;
       }
@@ -186,7 +186,7 @@ Accessible::Description(nsString& aDescription)
                            aDescription);
 
   if (aDescription.IsEmpty()) {
-    bool isXUL = mContent->IsXUL();
+    bool isXUL = mContent->IsXULElement();
     if (isXUL) {
       // Try XUL <description control="[id]">description text</description>
       XULDescriptionIterator iter(Document(), mContent);
@@ -199,14 +199,14 @@ Accessible::Description(nsString& aDescription)
 
     if (aDescription.IsEmpty()) {
       // Keep the Name() method logic.
-      if (mContent->IsHTML()) {
+      if (mContent->IsHTMLElement()) {
         mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::title, aDescription);
-      } else if (mContent->IsXUL()) {
+      } else if (mContent->IsXULElement()) {
         mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::tooltiptext, aDescription);
-      } else if (mContent->IsSVG()) {
+      } else if (mContent->IsSVGElement()) {
         for (nsIContent* childElm = mContent->GetFirstChild(); childElm;
              childElm = childElm->GetNextSibling()) {
-          if (childElm->IsSVG(nsGkAtoms::desc)) {
+          if (childElm->IsSVGElement(nsGkAtoms::desc)) {
             nsTextEquivUtils::AppendTextEquivFromContent(this, childElm,
                                                          &aDescription);
             break;
@@ -237,14 +237,14 @@ Accessible::AccessKey() const
     Accessible* label = nullptr;
 
     // Copy access key from label node.
-    if (mContent->IsHTML()) {
+    if (mContent->IsHTMLElement()) {
       // Unless it is labeled via an ancestor <label>, in which case that would
       // be redundant.
       HTMLLabelIterator iter(Document(), this,
                              HTMLLabelIterator::eSkipAncestorLabel);
       label = iter.Next();
 
-    } else if (mContent->IsXUL()) {
+    } else if (mContent->IsXULElement()) {
       XULLabelIterator iter(Document(), mContent);
       label = iter.Next();
     }
@@ -354,8 +354,7 @@ Accessible::VisibilityState()
     nsIFrame* parentFrame = curFrame->GetParent();
     nsDeckFrame* deckFrame = do_QueryFrame(parentFrame);
     if (deckFrame && deckFrame->GetSelectedBox() != curFrame) {
-      if (deckFrame->GetContent()->IsXUL() &&
-          deckFrame->GetContent()->Tag() == nsGkAtoms::tabpanels)
+      if (deckFrame->GetContent()->IsXULElement(nsGkAtoms::tabpanels))
         return states::OFFSCREEN;
 
       NS_NOTREACHED("Children of not selected deck panel are not accessible.");
@@ -435,7 +434,7 @@ Accessible::NativeState()
 
     // XXX we should look at layout for non XUL box frames, but need to decide
     // how that interacts with ARIA.
-    if (HasOwnContent() && mContent->IsXUL() && frame->IsBoxFrame()) {
+    if (HasOwnContent() && mContent->IsXULElement() && frame->IsBoxFrame()) {
       const nsStyleXUL* xulStyle = frame->StyleXUL();
       if (xulStyle && frame->IsBoxFrame()) {
         // In XUL all boxes are either vertical or horizontal
@@ -448,7 +447,7 @@ Accessible::NativeState()
   }
 
   // Check if a XUL element has the popup attribute (an attached popup menu).
-  if (HasOwnContent() && mContent->IsXUL() &&
+  if (HasOwnContent() && mContent->IsXULElement() &&
       mContent->HasAttr(kNameSpaceID_None, nsGkAtoms::popup))
     state |= states::HASPOPUP;
 
@@ -485,7 +484,7 @@ Accessible::NativeLinkState() const
 bool
 Accessible::NativelyUnavailable() const
 {
-  if (mContent->IsHTML())
+  if (mContent->IsHTMLElement())
     return mContent->AsElement()->State().HasState(NS_EVENT_STATE_DISABLED);
 
   return mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::disabled,
@@ -820,7 +819,7 @@ Accessible::XULElmName(DocAccessible* aDocument,
   nsIContent* parent =
     bindingParent? bindingParent->GetParent() : aElm->GetParent();
   while (parent) {
-    if (parent->Tag() == nsGkAtoms::toolbaritem &&
+    if (parent->IsXULElement(nsGkAtoms::toolbaritem) &&
         parent->GetAttr(kNameSpaceID_None, nsGkAtoms::title, aName)) {
       aName.CompressWhitespace();
       return;
@@ -1519,9 +1518,9 @@ Accessible::RelationByType(RelationType aType)
     case RelationType::LABELLED_BY: {
       Relation rel(new IDRefsIterator(mDoc, mContent,
                                       nsGkAtoms::aria_labelledby));
-      if (mContent->IsHTML()) {
+      if (mContent->IsHTMLElement()) {
         rel.AppendIter(new HTMLLabelIterator(Document(), this));
-      } else if (mContent->IsXUL()) {
+      } else if (mContent->IsXULElement()) {
         rel.AppendIter(new XULLabelIterator(Document(), mContent));
       }
 
@@ -1531,7 +1530,7 @@ Accessible::RelationByType(RelationType aType)
     case RelationType::LABEL_FOR: {
       Relation rel(new RelatedAccIterator(Document(), mContent,
                                           nsGkAtoms::aria_labelledby));
-      if (mContent->Tag() == nsGkAtoms::label && mContent->IsXUL())
+      if (mContent->IsXULElement(nsGkAtoms::label))
         rel.AppendIter(new IDRefsIterator(mDoc, mContent, nsGkAtoms::control));
 
       return rel;
@@ -1540,7 +1539,7 @@ Accessible::RelationByType(RelationType aType)
     case RelationType::DESCRIBED_BY: {
       Relation rel(new IDRefsIterator(mDoc, mContent,
                                       nsGkAtoms::aria_describedby));
-      if (mContent->IsXUL())
+      if (mContent->IsXULElement())
         rel.AppendIter(new XULDescriptionIterator(Document(), mContent));
 
       return rel;
@@ -1553,8 +1552,7 @@ Accessible::RelationByType(RelationType aType)
       // This affectively adds an optional control attribute to xul:description,
       // which only affects accessibility, by allowing the description to be
       // tied to a control.
-      if (mContent->Tag() == nsGkAtoms::description &&
-          mContent->IsXUL())
+      if (mContent->IsXULElement(nsGkAtoms::description))
         rel.AppendIter(new IDRefsIterator(mDoc, mContent,
                                           nsGkAtoms::control));
 
@@ -1639,7 +1637,7 @@ Accessible::RelationByType(RelationType aType)
       return Relation();
 
     case RelationType::DEFAULT_BUTTON: {
-      if (mContent->IsHTML()) {
+      if (mContent->IsHTMLElement()) {
         // HTML form controls implements nsIFormControl interface.
         nsCOMPtr<nsIFormControl> control(do_QueryInterface(mContent));
         if (control) {
@@ -1874,7 +1872,7 @@ Accessible::ARIAName(nsString& aName)
 ENameValueFlag
 Accessible::NativeName(nsString& aName)
 {
-  if (mContent->IsHTML()) {
+  if (mContent->IsHTMLElement()) {
     Accessible* label = nullptr;
     HTMLLabelIterator iter(Document(), this);
     while ((label = iter.Next())) {
@@ -1890,7 +1888,7 @@ Accessible::NativeName(nsString& aName)
     return aName.IsEmpty() ? eNameOK : eNameFromSubtree;
   }
 
-  if (mContent->IsXUL()) {
+  if (mContent->IsXULElement()) {
     XULElmName(mDoc, mContent, aName);
     if (!aName.IsEmpty())
       return eNameOK;
@@ -1899,12 +1897,12 @@ Accessible::NativeName(nsString& aName)
     return aName.IsEmpty() ? eNameOK : eNameFromSubtree;
   }
 
-  if (mContent->IsSVG()) {
+  if (mContent->IsSVGElement()) {
     // If user agents need to choose among multiple ‘desc’ or ‘title’ elements
     // for processing, the user agent shall choose the first one.
     for (nsIContent* childElm = mContent->GetFirstChild(); childElm;
          childElm = childElm->GetNextSibling()) {
-      if (childElm->IsSVG(nsGkAtoms::title)) {
+      if (childElm->IsSVGElement(nsGkAtoms::title)) {
         nsTextEquivUtils::AppendTextEquivFromContent(this, childElm, &aName);
         return eNameOK;
       }
@@ -2526,7 +2524,7 @@ Accessible::GetActionRule() const
     return eNoAction;
 
   // Return "click" action on elements that have an attached popup menu.
-  if (mContent->IsXUL())
+  if (mContent->IsXULElement())
     if (mContent->HasAttr(kNameSpaceID_None, nsGkAtoms::popup))
       return eClickAction;
 
diff --git a/accessible/generic/Accessible.h b/accessible/generic/Accessible.h
index d9d0dbab6d..d613d3f76e 100644
--- a/accessible/generic/Accessible.h
+++ b/accessible/generic/Accessible.h
@@ -565,8 +565,7 @@ public:
 
   inline bool IsAbbreviation() const
   {
-    return mContent->IsHTML() &&
-      (mContent->Tag() == nsGkAtoms::abbr || mContent->Tag() == nsGkAtoms::acronym);
+    return mContent->IsAnyOfHTMLElements(nsGkAtoms::abbr, nsGkAtoms::acronym);
   }
 
   bool IsApplication() const { return mType == eApplicationType; }
diff --git a/accessible/generic/DocAccessible.cpp b/accessible/generic/DocAccessible.cpp
index dfab3557ba..214d0b8796 100644
--- a/accessible/generic/DocAccessible.cpp
+++ b/accessible/generic/DocAccessible.cpp
@@ -94,7 +94,7 @@ DocAccessible::
   mPresShell->SetDocAccessible(this);
 
   // If this is a XUL Document, it should not implement nsHyperText
-  if (mDocumentNode && mDocumentNode->IsXUL())
+  if (mDocumentNode && mDocumentNode->IsXULDocument())
     mGenericTypes &= ~eHyperText;
 }
 
@@ -877,7 +877,8 @@ DocAccessible::AttributeChangedImpl(Accessible* aAccessible,
   }
 
   // ARIA or XUL selection
-  if ((aAccessible->GetContent()->IsXUL() && aAttribute == nsGkAtoms::selected) ||
+  if ((aAccessible->GetContent()->IsXULElement() &&
+       aAttribute == nsGkAtoms::selected) ||
       aAttribute == nsGkAtoms::aria_selected) {
     Accessible* widget =
       nsAccUtils::GetSelectableContainer(aAccessible, aAccessible->State());
@@ -1341,7 +1342,7 @@ DocAccessible::ProcessInvalidationList()
 Accessible*
 DocAccessible::GetAccessibleEvenIfNotInMap(nsINode* aNode) const
 {
-if (!aNode->IsContent() || !aNode->AsContent()->IsHTML(nsGkAtoms::area))
+if (!aNode->IsContent() || !aNode->AsContent()->IsHTMLElement(nsGkAtoms::area))
     return GetAccessible(aNode);
 
   // XXX Bug 135040, incorrect when multiple images use the same map.
@@ -1503,15 +1504,13 @@ DocAccessible::AddDependentIDsFor(dom::Element* aRelProviderElm,
       continue;
 
     if (relAttr == nsGkAtoms::_for) {
-      if (!aRelProviderElm->IsHTML() ||
-          (aRelProviderElm->Tag() != nsGkAtoms::label &&
-           aRelProviderElm->Tag() != nsGkAtoms::output))
+      if (!aRelProviderElm->IsAnyOfHTMLElements(nsGkAtoms::label,
+                                                nsGkAtoms::output))
         continue;
 
     } else if (relAttr == nsGkAtoms::control) {
-      if (!aRelProviderElm->IsXUL() ||
-          (aRelProviderElm->Tag() != nsGkAtoms::label &&
-           aRelProviderElm->Tag() != nsGkAtoms::description))
+      if (!aRelProviderElm->IsAnyOfXULElements(nsGkAtoms::label,
+                                               nsGkAtoms::description))
         continue;
     }
 
diff --git a/accessible/generic/HyperTextAccessible.cpp b/accessible/generic/HyperTextAccessible.cpp
index d54ac9e553..ee9b73e5ff 100644
--- a/accessible/generic/HyperTextAccessible.cpp
+++ b/accessible/generic/HyperTextAccessible.cpp
@@ -59,34 +59,34 @@ NS_IMPL_ISUPPORTS_INHERITED0(HyperTextAccessible, Accessible)
 role
 HyperTextAccessible::NativeRole()
 {
-  nsIAtom *tag = mContent->Tag();
-
-  if (tag == nsGkAtoms::dd)
+  if (mContent->IsHTMLElement(nsGkAtoms::dd))
     return roles::DEFINITION;
 
-  if (tag == nsGkAtoms::form)
+  if (mContent->IsHTMLElement(nsGkAtoms::form))
     return roles::FORM;
 
-  if (tag == nsGkAtoms::blockquote || tag == nsGkAtoms::div ||
-      tag == nsGkAtoms::section || tag == nsGkAtoms::nav)
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::blockquote,
+                                    nsGkAtoms::div,
+                                    nsGkAtoms::section,
+                                    nsGkAtoms::nav))
     return roles::SECTION;
 
-  if (tag == nsGkAtoms::h1 || tag == nsGkAtoms::h2 ||
-      tag == nsGkAtoms::h3 || tag == nsGkAtoms::h4 ||
-      tag == nsGkAtoms::h5 || tag == nsGkAtoms::h6)
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::h1, nsGkAtoms::h2,
+                                    nsGkAtoms::h3, nsGkAtoms::h4,
+                                    nsGkAtoms::h5, nsGkAtoms::h6))
     return roles::HEADING;
 
-  if (tag == nsGkAtoms::article)
+  if (mContent->IsHTMLElement(nsGkAtoms::article))
     return roles::DOCUMENT;
 
   // Deal with html landmark elements
-  if (tag == nsGkAtoms::header)
+  if (mContent->IsHTMLElement(nsGkAtoms::header))
     return roles::HEADER;
 
-  if (tag == nsGkAtoms::footer)
+  if (mContent->IsHTMLElement(nsGkAtoms::footer))
     return roles::FOOTER;
 
-  if (tag == nsGkAtoms::aside)
+  if (mContent->IsHTMLElement(nsGkAtoms::aside))
     return roles::NOTE;
 
   // Treat block frames as paragraphs
@@ -105,7 +105,7 @@ HyperTextAccessible::NativeState()
   if (mContent->AsElement()->State().HasState(NS_EVENT_STATE_MOZ_READWRITE)) {
     states |= states::EDITABLE;
 
-  } else if (mContent->Tag() == nsGkAtoms::article) {
+  } else if (mContent->IsHTMLElement(nsGkAtoms::article)) {
     // We want <article> to behave like a document in terms of readonly state.
     states |= states::READONLY;
   }
@@ -294,7 +294,7 @@ HyperTextAccessible::DOMPointToOffset(nsINode* aNode, int32_t aNodeOffset,
   Accessible* descendant = nullptr;
   if (findNode) {
     nsCOMPtr<nsIContent> findContent(do_QueryInterface(findNode));
-    if (findContent && findContent->IsHTML() &&
+    if (findContent && findContent->IsHTMLElement() &&
         findContent->NodeInfo()->Equals(nsGkAtoms::br) &&
         findContent->AttrValueIs(kNameSpaceID_None,
                                  nsGkAtoms::mozeditorbogusnode,
@@ -900,18 +900,17 @@ HyperTextAccessible::DefaultTextAttributes()
 int32_t
 HyperTextAccessible::GetLevelInternal()
 {
-  nsIAtom *tag = mContent->Tag();
-  if (tag == nsGkAtoms::h1)
+  if (mContent->IsHTMLElement(nsGkAtoms::h1))
     return 1;
-  if (tag == nsGkAtoms::h2)
+  if (mContent->IsHTMLElement(nsGkAtoms::h2))
     return 2;
-  if (tag == nsGkAtoms::h3)
+  if (mContent->IsHTMLElement(nsGkAtoms::h3))
     return 3;
-  if (tag == nsGkAtoms::h4)
+  if (mContent->IsHTMLElement(nsGkAtoms::h4))
     return 4;
-  if (tag == nsGkAtoms::h5)
+  if (mContent->IsHTMLElement(nsGkAtoms::h5))
     return 5;
-  if (tag == nsGkAtoms::h6)
+  if (mContent->IsHTMLElement(nsGkAtoms::h6))
     return 6;
 
   return AccessibleWrap::GetLevelInternal();
@@ -944,14 +943,13 @@ HyperTextAccessible::NativeAttributes()
   if (!HasOwnContent())
     return attributes.forget();
 
-  nsIAtom* tag = mContent->Tag();
-  if (tag == nsGkAtoms::section)  {
+  if (mContent->IsHTMLElement(nsGkAtoms::section)) {
     nsAccUtils::SetAccAttr(attributes, nsGkAtoms::xmlroles,
                            NS_LITERAL_STRING("region"));
-  } else if (tag == nsGkAtoms::article) {
+  } else if (mContent->IsHTMLElement(nsGkAtoms::article)) {
     nsAccUtils::SetAccAttr(attributes, nsGkAtoms::xmlroles,
                            NS_LITERAL_STRING("article"));
-  } else if (tag == nsGkAtoms::time) {
+  } else if (mContent->IsHTMLElement(nsGkAtoms::time)) {
     nsAccUtils::SetAccAttr(attributes, nsGkAtoms::xmlroles,
                            NS_LITERAL_STRING("time"));
 
@@ -970,38 +968,42 @@ HyperTextAccessible::LandmarkRole() const
 {
   // For the html landmark elements we expose them like we do ARIA landmarks to
   // make AT navigation schemes "just work".
-  nsIAtom* tag = mContent->Tag();
-  if (tag == nsGkAtoms::nav)
+  if (mContent->IsHTMLElement(nsGkAtoms::nav)) {
     return nsGkAtoms::navigation;
+  }
 
-  if (tag == nsGkAtoms::header || tag == nsGkAtoms::footer) {
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::header,
+                                    nsGkAtoms::footer)) {
     // Only map header and footer if they are not descendants of an article
     // or section tag.
     nsIContent* parent = mContent->GetParent();
     while (parent) {
-      if (parent->Tag() == nsGkAtoms::article ||
-          parent->Tag() == nsGkAtoms::section)
+      if (parent->IsAnyOfHTMLElements(nsGkAtoms::article, nsGkAtoms::section)) {
         break;
+      }
       parent = parent->GetParent();
     }
 
     // No article or section elements found.
     if (!parent) {
-      if (tag == nsGkAtoms::header)
+      if (mContent->IsHTMLElement(nsGkAtoms::header)) {
         return nsGkAtoms::banner;
+      }
 
-      if (tag == nsGkAtoms::footer) {
+      if (mContent->IsHTMLElement(nsGkAtoms::footer)) {
         return nsGkAtoms::contentinfo;
       }
     }
     return nullptr;
   }
 
-  if (tag == nsGkAtoms::aside)
+  if (mContent->IsHTMLElement(nsGkAtoms::aside)) {
     return nsGkAtoms::complementary;
+  }
 
-  if (tag == nsGkAtoms::main)
+  if (mContent->IsHTMLElement(nsGkAtoms::main)) {
     return nsGkAtoms::main;
+  }
 
   return nullptr;
 }
@@ -1692,7 +1694,7 @@ HyperTextAccessible::NativeName(nsString& aName)
 {
   // Check @alt attribute for invalid img elements.
   bool hasImgAlt = false;
-  if (mContent->IsHTML(nsGkAtoms::img)) {
+  if (mContent->IsHTMLElement(nsGkAtoms::img)) {
     hasImgAlt = mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::alt, aName);
     if (!aName.IsEmpty())
       return eNameOK;
diff --git a/accessible/generic/ImageAccessible.cpp b/accessible/generic/ImageAccessible.cpp
index 50efc7720f..8c990c226a 100644
--- a/accessible/generic/ImageAccessible.cpp
+++ b/accessible/generic/ImageAccessible.cpp
@@ -200,7 +200,8 @@ ImageAccessible::GetLongDescURI() const
   if (document) {
     IDRefsIterator iter(document, mContent, nsGkAtoms::aria_describedby);
     while (nsIContent* target = iter.NextElem()) {
-      if ((target->IsHTML(nsGkAtoms::a) || target->IsHTML(nsGkAtoms::area)) &&
+      if ((target->IsHTMLElement(nsGkAtoms::a) ||
+           target->IsHTMLElement(nsGkAtoms::area)) &&
           target->HasAttr(kNameSpaceID_None, nsGkAtoms::href)) {
         nsGenericHTMLElement* element =
           nsGenericHTMLElement::FromContent(target);
diff --git a/accessible/generic/RootAccessible.cpp b/accessible/generic/RootAccessible.cpp
index e98e7636f2..801e473ce5 100644
--- a/accessible/generic/RootAccessible.cpp
+++ b/accessible/generic/RootAccessible.cpp
@@ -92,8 +92,8 @@ RootAccessible::NativeRole()
 {
   // If it's a <dialog> or <wizard>, use roles::DIALOG instead
   dom::Element* rootElm = mDocumentNode->GetRootElement();
-  if (rootElm && (rootElm->Tag() == nsGkAtoms::dialog ||
-                  rootElm->Tag() == nsGkAtoms::wizard))
+  if (rootElm && rootElm->IsAnyOfXULElements(nsGkAtoms::dialog,
+                                             nsGkAtoms::wizard))
     return roles::DIALOG;
 
   return DocAccessibleWrap::NativeRole();
diff --git a/accessible/html/HTMLFormControlAccessible.cpp b/accessible/html/HTMLFormControlAccessible.cpp
index 8846eed781..9d655bbee6 100644
--- a/accessible/html/HTMLFormControlAccessible.cpp
+++ b/accessible/html/HTMLFormControlAccessible.cpp
@@ -250,7 +250,7 @@ HTMLButtonAccessible::NativeName(nsString& aName)
   // type="submit"/"reset"/"image" elements.
 
   ENameValueFlag nameFlag = Accessible::NativeName(aName);
-  if (!aName.IsEmpty() || mContent->Tag() != nsGkAtoms::input ||
+  if (!aName.IsEmpty() || !mContent->IsHTMLElement(nsGkAtoms::input) ||
       !mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                              nsGkAtoms::image, eCaseMatters))
     return nameFlag;
diff --git a/accessible/html/HTMLListAccessible.cpp b/accessible/html/HTMLListAccessible.cpp
index 182fc13c4d..533a38ea80 100644
--- a/accessible/html/HTMLListAccessible.cpp
+++ b/accessible/html/HTMLListAccessible.cpp
@@ -26,7 +26,7 @@ NS_IMPL_ISUPPORTS_INHERITED0(HTMLListAccessible, HyperTextAccessible)
 role
 HTMLListAccessible::NativeRole()
 {
-  if (mContent->Tag() == nsGkAtoms::dl)
+  if (mContent->IsHTMLElement(nsGkAtoms::dl))
     return roles::DEFINITION_LIST;
 
   return roles::LIST;
@@ -69,7 +69,7 @@ HTMLLIAccessible::Shutdown()
 role
 HTMLLIAccessible::NativeRole()
 {
-  if (mContent->Tag() == nsGkAtoms::dt)
+  if (mContent->IsHTMLElement(nsGkAtoms::dt))
     return roles::TERM;
 
   return roles::LISTITEM;
diff --git a/accessible/html/HTMLSelectAccessible.cpp b/accessible/html/HTMLSelectAccessible.cpp
index accdd02bc4..5a905a373c 100644
--- a/accessible/html/HTMLSelectAccessible.cpp
+++ b/accessible/html/HTMLSelectAccessible.cpp
@@ -126,13 +126,12 @@ HTMLSelectListAccessible::CacheChildren()
   // a flat tree under the Select List.
   for (nsIContent* childContent = mContent->GetFirstChild(); childContent;
        childContent = childContent->GetNextSibling()) {
-    if (!childContent->IsHTML()) {
+    if (!childContent->IsHTMLElement()) {
       continue;
     }
 
-    nsIAtom* tag = childContent->Tag();
-    if (tag == nsGkAtoms::option ||
-        tag == nsGkAtoms::optgroup) {
+    if (childContent->IsAnyOfHTMLElements(nsGkAtoms::option,
+                                          nsGkAtoms::optgroup)) {
 
       // Get an accessible for option or optgroup and cache it.
       nsRefPtr<Accessible> accessible =
diff --git a/accessible/html/HTMLTableAccessible.cpp b/accessible/html/HTMLTableAccessible.cpp
index b897803bdd..e6ae5d43a2 100644
--- a/accessible/html/HTMLTableAccessible.cpp
+++ b/accessible/html/HTMLTableAccessible.cpp
@@ -929,7 +929,7 @@ HTMLTableAccessible::IsProbablyLayoutTable()
     RETURN_LAYOUT_ANSWER(false, "Has role attribute, weak role, and role is table");
   }
 
-  if (mContent->Tag() != nsGkAtoms::table)
+  if (!mContent->IsHTMLElement(nsGkAtoms::table))
     RETURN_LAYOUT_ANSWER(true, "table built by CSS display:table style");
 
   // Check if datatable attribute has "0" value.
@@ -951,24 +951,24 @@ HTMLTableAccessible::IsProbablyLayoutTable()
 
   for (nsIContent* childElm = mContent->GetFirstChild(); childElm;
        childElm = childElm->GetNextSibling()) {
-    if (!childElm->IsHTML())
+    if (!childElm->IsHTMLElement())
       continue;
 
-    if (childElm->Tag() == nsGkAtoms::col ||
-        childElm->Tag() == nsGkAtoms::colgroup ||
-        childElm->Tag() == nsGkAtoms::tfoot ||
-        childElm->Tag() == nsGkAtoms::thead) {
+    if (childElm->IsAnyOfHTMLElements(nsGkAtoms::col,
+                                      nsGkAtoms::colgroup,
+                                      nsGkAtoms::tfoot,
+                                      nsGkAtoms::thead)) {
       RETURN_LAYOUT_ANSWER(false,
                            "Has col, colgroup, tfoot or thead -- legitimate table structures");
     }
 
-    if (childElm->Tag() == nsGkAtoms::tbody) {
+    if (childElm->IsHTMLElement(nsGkAtoms::tbody)) {
       for (nsIContent* rowElm = childElm->GetFirstChild(); rowElm;
            rowElm = rowElm->GetNextSibling()) {
-        if (rowElm->IsHTML() && rowElm->Tag() == nsGkAtoms::tr) {
+        if (rowElm->IsHTMLElement(nsGkAtoms::tr)) {
           for (nsIContent* cellElm = rowElm->GetFirstChild(); cellElm;
                cellElm = cellElm->GetNextSibling()) {
-            if (cellElm->IsHTML()) {
+            if (cellElm->IsHTMLElement()) {
 
               if (cellElm->NodeInfo()->Equals(nsGkAtoms::th)) {
                 RETURN_LAYOUT_ANSWER(false,
diff --git a/accessible/windows/msaa/AccessibleWrap.cpp b/accessible/windows/msaa/AccessibleWrap.cpp
index 46f2d4d5ff..d6934663a0 100644
--- a/accessible/windows/msaa/AccessibleWrap.cpp
+++ b/accessible/windows/msaa/AccessibleWrap.cpp
@@ -1214,7 +1214,7 @@ AccessibleWrap::HandleAccEvent(AccEvent* aEvent)
   nsAutoCString id;
   nsIContent* cnt = accessible->GetContent();
   if (cnt) {
-    cnt->Tag()->ToString(tag);
+    cnt->NodeInfo()->NameAtom()->ToString(tag);
     nsIAtom* aid = cnt->GetID();
     if (aid)
       aid->ToUTF8String(id);
diff --git a/accessible/xul/XULElementAccessibles.cpp b/accessible/xul/XULElementAccessibles.cpp
index 58843f7d7b..629545628c 100644
--- a/accessible/xul/XULElementAccessibles.cpp
+++ b/accessible/xul/XULElementAccessibles.cpp
@@ -92,7 +92,7 @@ XULLabelAccessible::RelationByType(RelationType aType)
   if (aType == RelationType::LABEL_FOR) {
     // Caption is the label for groupbox
     nsIContent* parent = mContent->GetFlattenedTreeParent();
-    if (parent && parent->Tag() == nsGkAtoms::caption) {
+    if (parent && parent->IsXULElement(nsGkAtoms::caption)) {
       Accessible* parent = Parent();
       if (parent && parent->Role() == roles::GROUPING)
         rel.AppendTarget(parent);
diff --git a/accessible/xul/XULFormControlAccessible.cpp b/accessible/xul/XULFormControlAccessible.cpp
index 98bda6ad46..8eb104e2a1 100644
--- a/accessible/xul/XULFormControlAccessible.cpp
+++ b/accessible/xul/XULFormControlAccessible.cpp
@@ -182,7 +182,7 @@ XULButtonAccessible::IsAcceptableChild(Accessible* aPossibleChild) const
   // Button type="menu-button" contains a real button. Get an accessible
   // for it. Ignore dropmarker button which is placed as a last child.
   if (role != roles::PUSHBUTTON ||
-      aPossibleChild->GetContent()->Tag() == nsGkAtoms::dropMarker)
+      aPossibleChild->GetContent()->IsXULElement(nsGkAtoms::dropMarker))
     return false;
 
   return mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
@@ -569,9 +569,10 @@ bool
 XULToolbarButtonAccessible::IsSeparator(Accessible* aAccessible)
 {
   nsIContent* content = aAccessible->GetContent();
-  return content && ((content->Tag() == nsGkAtoms::toolbarseparator) ||
-                     (content->Tag() == nsGkAtoms::toolbarspacer) ||
-                     (content->Tag() == nsGkAtoms::toolbarspring)); }
+  return content && content->IsAnyOfXULElements(nsGkAtoms::toolbarseparator,
+                                                nsGkAtoms::toolbarspacer,
+                                                nsGkAtoms::toolbarspring);
+}
 
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 04060258a2..e9c96de3e2 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -352,11 +352,7 @@ IsElementAnchor(nsIContent* aContent)
 {
   // Make sure we are dealing with either an <A> or <AREA> element in the HTML
   // or XHTML namespace.
-  if (!aContent->IsHTML()) {
-    return false;
-  }
-  nsIAtom* nameAtom = aContent->Tag();
-  return nameAtom == nsGkAtoms::a || nameAtom == nsGkAtoms::area;
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::a, nsGkAtoms::area);
 }
 
 static void
@@ -2656,7 +2652,7 @@ nsDocShell::GetFullscreenAllowed(bool* aFullscreenAllowed)
   }
   nsCOMPtr<Element> frameElement = win->GetFrameElementInternal();
   if (frameElement &&
-      frameElement->IsHTML(nsGkAtoms::iframe) &&
+      frameElement->IsHTMLElement(nsGkAtoms::iframe) &&
       !frameElement->HasAttr(kNameSpaceID_None, nsGkAtoms::allowfullscreen) &&
       !frameElement->HasAttr(kNameSpaceID_None, nsGkAtoms::mozallowfullscreen)) {
     return NS_OK;
@@ -13632,7 +13628,8 @@ nsDocShell::OnLinkClickSync(nsIContent* aContent,
   // XXX When the linking node was HTMLFormElement, it is synchronous event.
   //     That is, the caller of this method is not |OnLinkClickEvent::Run()|
   //     but |HTMLFormElement::SubmitSubmission(...)|.
-  if (nsGkAtoms::form == aContent->Tag() && ShouldBlockLoadingForBackButton()) {
+  if (aContent->IsHTMLElement(nsGkAtoms::form) &&
+      ShouldBlockLoadingForBackButton()) {
     return NS_OK;
   }
 
@@ -13662,7 +13659,7 @@ nsDocShell::OnLinkClickSync(nsIContent* aContent,
 
   uint32_t flags = INTERNAL_LOAD_FLAGS_NONE;
   if (IsElementAnchor(aContent)) {
-    MOZ_ASSERT(aContent->IsHTML());
+    MOZ_ASSERT(aContent->IsHTMLElement());
     nsAutoString referrer;
     aContent->GetAttr(kNameSpaceID_None, nsGkAtoms::rel, referrer);
     nsWhitespaceTokenizerTemplate<nsContentUtils::IsHTMLWhitespace> tok(referrer);
diff --git a/dom/base/Attr.cpp b/dom/base/Attr.cpp
index 50a1993268..b67b8de77a 100644
--- a/dom/base/Attr.cpp
+++ b/dom/base/Attr.cpp
@@ -167,7 +167,7 @@ Attr::GetNameAtom(nsIContent* aContent)
   if (!mNsAware &&
       mNodeInfo->NamespaceID() == kNameSpaceID_None &&
       aContent->IsInHTMLDocument() &&
-      aContent->IsHTML()) {
+      aContent->IsHTMLElement()) {
     nsString name;
     mNodeInfo->GetName(name);
     nsAutoString lowercaseName;
diff --git a/dom/base/ChildIterator.cpp b/dom/base/ChildIterator.cpp
index 032aeb90a1..2f2fd99376 100644
--- a/dom/base/ChildIterator.cpp
+++ b/dom/base/ChildIterator.cpp
@@ -58,7 +58,7 @@ GetMatchedNodesForPoint(nsIContent* aContent)
   }
 
   // Web components case
-  MOZ_ASSERT(aContent->IsHTML(nsGkAtoms::content));
+  MOZ_ASSERT(aContent->IsHTMLElement(nsGkAtoms::content));
   return MatchedNodes(static_cast<HTMLContentElement*>(aContent));
 }
 
diff --git a/dom/base/DirectionalityUtils.cpp b/dom/base/DirectionalityUtils.cpp
index 8b8ee4c186..0a92ae801e 100644
--- a/dom/base/DirectionalityUtils.cpp
+++ b/dom/base/DirectionalityUtils.cpp
@@ -236,7 +236,7 @@ static bool
 DoesNotParticipateInAutoDirection(const Element* aElement)
 {
   mozilla::dom::NodeInfo* nodeInfo = aElement->NodeInfo();
-  return (!aElement->IsHTML() ||
+  return (!aElement->IsHTMLElement() ||
           nodeInfo->Equals(nsGkAtoms::script) ||
           nodeInfo->Equals(nsGkAtoms::style) ||
           nodeInfo->Equals(nsGkAtoms::textarea) ||
@@ -249,7 +249,7 @@ IsBdiWithoutDirAuto(const Element* aElement)
   // We are testing for bdi elements without explicit dir="auto", so we can't
   // use the HasDirAuto() flag, since that will return true for bdi element with
   // no dir attribute or an invalid dir attribute
-  return (aElement->IsHTML(nsGkAtoms::bdi) &&
+  return (aElement->IsHTMLElement(nsGkAtoms::bdi) &&
           (!aElement->HasValidDir() || aElement->HasFixedDir()));
 }
 
@@ -931,7 +931,7 @@ void
 OnSetDirAttr(Element* aElement, const nsAttrValue* aNewValue,
              bool hadValidDir, bool hadDirAuto, bool aNotify)
 {
-  if (aElement->IsHTML(nsGkAtoms::input)) {
+  if (aElement->IsHTMLElement(nsGkAtoms::input)) {
     return;
   }
 
@@ -986,7 +986,7 @@ SetDirOnBind(mozilla::dom::Element* aElement, nsIContent* aParent)
   // Set the AncestorHasDirAuto flag, unless this element shouldn't affect
   // ancestors that have dir=auto
   if (!DoesNotParticipateInAutoDirection(aElement) &&
-      !aElement->IsHTML(nsGkAtoms::bdi) &&
+      !aElement->IsHTMLElement(nsGkAtoms::bdi) &&
       aParent && aParent->NodeOrAncestorHasDirAuto()) {
     aElement->SetAncestorHasDirAuto();
 
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 0076e1a568..5c5a398946 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -158,7 +158,7 @@ nsIContent::DoGetClasses() const
   MOZ_ASSERT(HasFlag(NODE_MAY_HAVE_CLASS), "Unexpected call");
   MOZ_ASSERT(IsElement(), "Only elements can have classes");
 
-  if (IsSVG()) {
+  if (IsSVGElement()) {
     const nsAttrValue* animClass =
       static_cast<const nsSVGElement*>(this)->GetAnimatedClassName();
     if (animClass) {
@@ -387,10 +387,10 @@ Element::GetBindingURL(nsIDocument *aDocument, css::URLValue **aResult)
   // otherwise, don't do anything else here unless we're dealing with
   // XUL or an HTML element that may have a plugin-related overlay
   // (i.e. object, embed, or applet).
-  bool isXULorPluginElement = (IsXUL() ||
-                               IsHTML(nsGkAtoms::object) ||
-                               IsHTML(nsGkAtoms::embed) ||
-                               IsHTML(nsGkAtoms::applet));
+  bool isXULorPluginElement = (IsXULElement() ||
+                               IsHTMLElement(nsGkAtoms::object) ||
+                               IsHTMLElement(nsGkAtoms::embed) ||
+                               IsHTMLElement(nsGkAtoms::applet));
   nsIPresShell *shell = aDocument->GetShell();
   if (!shell || GetPrimaryFrame() || !isXULorPluginElement) {
     *aResult = nullptr;
@@ -557,7 +557,7 @@ nsIScrollableFrame*
 Element::GetScrollFrame(nsIFrame **aStyledFrame, bool aFlushLayout)
 {
   // it isn't clear what to return for SVG nodes, so just return nothing
-  if (IsSVG()) {
+  if (IsSVGElement()) {
     if (aStyledFrame) {
       *aStyledFrame = nullptr;
     }
@@ -837,7 +837,7 @@ static nsSize GetScrollRectSizeForOverflowVisibleFrame(nsIFrame* aFrame)
 int32_t
 Element::ScrollHeight()
 {
-  if (IsSVG())
+  if (IsSVGElement())
     return 0;
 
   nsIScrollableFrame* sf = GetScrollFrame();
@@ -854,7 +854,7 @@ Element::ScrollHeight()
 int32_t
 Element::ScrollWidth()
 {
-  if (IsSVG())
+  if (IsSVGElement())
     return 0;
 
   nsIScrollableFrame* sf = GetScrollFrame();
@@ -938,7 +938,7 @@ Element::AddToIdTable(nsIAtom* aId)
     containingShadow->AddToIdTable(this, aId);
   } else {
     nsIDocument* doc = GetUncomposedDoc();
-    if (doc && (!IsInAnonymousSubtree() || doc->IsXUL())) {
+    if (doc && (!IsInAnonymousSubtree() || doc->IsXULDocument())) {
       doc->AddToIdTable(this, aId);
     }
   }
@@ -961,7 +961,7 @@ Element::RemoveFromIdTable()
     }
   } else {
     nsIDocument* doc = GetUncomposedDoc();
-    if (doc && (!IsInAnonymousSubtree() || doc->IsXUL())) {
+    if (doc && (!IsInAnonymousSubtree() || doc->IsXULDocument())) {
       doc->RemoveFromIdTable(this, id);
     }
   }
@@ -1128,12 +1128,12 @@ Element::GetAttribute(const nsAString& aName, DOMString& aReturn)
 {
   const nsAttrValue* val =
     mAttrsAndChildren.GetAttr(aName,
-                              IsHTML() && IsInHTMLDocument() ?
+                              IsHTMLElement() && IsInHTMLDocument() ?
                                 eIgnoreCase : eCaseMatters);
   if (val) {
     val->ToString(aReturn);
   } else {
-    if (IsXUL()) {
+    if (IsXULElement()) {
       // XXX should be SetDOMStringToNull(aReturn);
       // See bug 232598
       // aReturn is already empty
@@ -1157,7 +1157,7 @@ Element::SetAttribute(const nsAString& aName,
     }
 
     nsCOMPtr<nsIAtom> nameAtom;
-    if (IsHTML() && IsInHTMLDocument()) {
+    if (IsHTMLElement() && IsInHTMLDocument()) {
       nsAutoString lower;
       nsContentUtils::ASCIIToLower(aName, lower);
       nameAtom = do_GetAtom(lower);
@@ -1556,7 +1556,7 @@ Element::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
   // This has to be here, rather than in nsGenericHTMLElement::BindToTree,
   //  because it has to happen after updating the parent pointer, but before
   //  recursively binding the kids.
-  if (IsHTML()) {
+  if (IsHTMLElement()) {
     SetDirOnBind(this, aParent);
   }
 
@@ -1622,7 +1622,7 @@ Element::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
     AddToIdTable(DoGetID());
   }
 
-  if (MayHaveStyle() && !IsXUL()) {
+  if (MayHaveStyle() && !IsXULElement()) {
     // XXXbz if we already have a style attr parsed, this won't do
     // anything... need to fix that.
     // If MayHaveStyle() is true, we must be an nsStyledElement
@@ -1812,7 +1812,7 @@ Element::UnbindFromTree(bool aDeep, bool aNullParent)
   // This has to be here, rather than in nsGenericHTMLElement::UnbindFromTree, 
   //  because it has to happen after unsetting the parent pointer, but before
   //  recursively unbinding the kids.
-  if (IsHTML()) {
+  if (IsHTMLElement()) {
     ResetDir(this);
   }
 
@@ -2317,7 +2317,7 @@ Element::SetAttrAndNotify(int32_t aNamespaceID,
 
   if (aNamespaceID == kNameSpaceID_None) {
     if (aName == nsGkAtoms::dir) {
-      hadValidDir = HasValidDir() || IsHTML(nsGkAtoms::bdi);
+      hadValidDir = HasValidDir() || IsHTMLElement(nsGkAtoms::bdi);
       hadDirAuto = HasDirAuto(); // already takes bdi into account
     }
 
@@ -2554,7 +2554,7 @@ Element::UnsetAttr(int32_t aNameSpaceID, nsIAtom* aName,
   bool hadDirAuto = false;
 
   if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::dir) {
-    hadValidDir = HasValidDir() || IsHTML(nsGkAtoms::bdi);
+    hadValidDir = HasValidDir() || IsHTMLElement(nsGkAtoms::bdi);
     hadDirAuto = HasDirAuto(); // already takes bdi into account
   }
 
@@ -3294,12 +3294,12 @@ Element::SetOuterHTML(const nsAString& aOuterHTML, ErrorResult& aError)
     return;
   }
 
-  if (OwnerDoc()->IsHTML()) {
+  if (OwnerDoc()->IsHTMLDocument()) {
     nsIAtom* localName;
     int32_t namespaceID;
     if (parent->IsElement()) {
-      localName = static_cast<nsIContent*>(parent.get())->Tag();
-      namespaceID = static_cast<nsIContent*>(parent.get())->GetNameSpaceID();
+      localName = parent->NodeInfo()->NameAtom();
+      namespaceID = parent->NodeInfo()->NamespaceID();
     } else {
       NS_ASSERTION(parent->NodeType() == nsIDOMNode::DOCUMENT_FRAGMENT_NODE,
         "How come the parent isn't a document, a fragment or an element?");
@@ -3391,13 +3391,13 @@ Element::InsertAdjacentHTML(const nsAString& aPosition, const nsAString& aText,
   mozAutoSubtreeModified subtree(doc, nullptr);
 
   // Parse directly into destination if possible
-  if (doc->IsHTML() && !OwnerDoc()->MayHaveDOMMutationObservers() &&
+  if (doc->IsHTMLDocument() && !OwnerDoc()->MayHaveDOMMutationObservers() &&
       (position == eBeforeEnd ||
        (position == eAfterEnd && !GetNextSibling()) ||
        (position == eAfterBegin && !GetFirstChild()))) {
     int32_t oldChildCount = destination->GetChildCount();
     int32_t contextNs = destination->GetNameSpaceID();
-    nsIAtom* contextLocal = destination->Tag();
+    nsIAtom* contextLocal = destination->NodeInfo()->NameAtom();
     if (contextLocal == nsGkAtoms::html && contextNs == kNameSpaceID_XHTML) {
       // For compat with IE6 through IE9. Willful violation of HTML5 as of
       // 2011-04-06. CreateContextualFragment does the same already.
diff --git a/dom/base/Element.h b/dom/base/Element.h
index c3acb45d41..dd391d0390 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -351,7 +351,7 @@ public:
   // false
   inline bool HasDirAuto() const {
     return (!HasFixedDir() &&
-            (HasValidDir() || IsHTML(nsGkAtoms::bdi)));
+            (HasValidDir() || IsHTMLElement(nsGkAtoms::bdi)));
   }
 
   Directionality GetComputedDirectionality() const;
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 4f04204231..20372ed9ff 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -306,7 +306,7 @@ nsIContent::GetBaseURI(bool aTryUseXHRDocBaseURI) const
   const nsIContent *elem = this;
   do {
     // First check for SVG specialness (why is this SVG specific?)
-    if (elem->IsSVG()) {
+    if (elem->IsSVGElement()) {
       nsIContent* bindingParent = elem->GetBindingParent();
       if (bindingParent) {
         nsXBLBinding* binding = bindingParent->GetXBLBinding();
@@ -731,10 +731,10 @@ nsIContent::PreHandleEvent(EventChainPreVisitor& aVisitor)
                 do_QueryInterface(aVisitor.mEvent->originalTarget);
               nsAutoString ot, ct, rt;
               if (originalTarget) {
-                originalTarget->Tag()->ToString(ot);
+                originalTarget->NodeInfo()->NameAtom()->ToString(ot);
               }
-              Tag()->ToString(ct);
-              relatedTarget->Tag()->ToString(rt);
+              NodeInfo()->NameAtom()->ToString(ct);
+              relatedTarget->NodeInfo()->NameAtom()->ToString(rt);
               printf("Stopping %s propagation:"
                      "\n\toriginalTarget=%s \n\tcurrentTarget=%s %s"
                      "\n\trelatedTarget=%s %s \n%s",
@@ -1347,7 +1347,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FragmentOrElement)
   // which is dispatched in UnbindFromTree.
 
   if (tmp->HasProperties()) {
-    if (tmp->IsHTML() || tmp->IsSVG()) {
+    if (tmp->IsHTMLElement() || tmp->IsSVGElement()) {
       nsIAtom*** props = Element::HTMLSVGPropertiesToTraverseAndUnlink();
       for (uint32_t i = 0; props[i]; ++i) {
         tmp->DeleteProperty(*props[i]);
@@ -1389,7 +1389,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FragmentOrElement)
   {
     nsDOMSlots *slots = tmp->GetExistingDOMSlots();
     if (slots) {
-      slots->Unlink(tmp->IsXUL());
+      slots->Unlink(tmp->IsXULElement());
     }
   }
 
@@ -1899,7 +1899,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(FragmentOrElement)
   tmp->OwnerDoc()->BindingManager()->Traverse(tmp, cb);
 
   if (tmp->HasProperties()) {
-    if (tmp->IsHTML() || tmp->IsSVG()) {
+    if (tmp->IsHTMLElement() || tmp->IsSVGElement()) {
       nsIAtom*** props = Element::HTMLSVGPropertiesToTraverseAndUnlink();
       for (uint32_t i = 0; props[i]; ++i) {
         nsISupports* property =
@@ -1934,7 +1934,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(FragmentOrElement)
   {
     nsDOMSlots *slots = tmp->GetExistingDOMSlots();
     if (slots) {
-      slots->Traverse(cb, tmp->IsXUL());
+      slots->Traverse(cb, tmp->IsXULElement());
     }
   }
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
@@ -2428,11 +2428,12 @@ AppendEncodedAttributeValue(nsAutoString* aValue, StringBuilder& aBuilder)
 static void
 StartElement(Element* aContent, StringBuilder& aBuilder)
 {
-  nsIAtom* localName = aContent->Tag();
+  nsIAtom* localName = aContent->NodeInfo()->NameAtom();
   int32_t tagNS = aContent->GetNameSpaceID();
 
   aBuilder.Append("<");
-  if (aContent->IsHTML() || aContent->IsSVG() || aContent->IsMathML()) {
+  if (aContent->IsHTMLElement() || aContent->IsSVGElement() ||
+      aContent->IsMathMLElement()) {
     aBuilder.Append(localName);
   } else {
     aBuilder.Append(aContent->NodeName());
@@ -2497,7 +2498,7 @@ StartElement(Element* aContent, StringBuilder& aBuilder)
   // pre/textarea/listing is a textnode and starts with a \n.
   // But because browsers haven't traditionally had that behavior,
   // we're not changing our behavior either - yet.
-  if (aContent->IsHTML()) {
+  if (aContent->IsHTMLElement()) {
     if (localName == nsGkAtoms::pre || localName == nsGkAtoms::textarea ||
         localName == nsGkAtoms::listing) {
       nsIContent* fc = aContent->GetFirstChild();
@@ -2516,7 +2517,7 @@ StartElement(Element* aContent, StringBuilder& aBuilder)
 static inline bool
 ShouldEscape(nsIContent* aParent)
 {
-  if (!aParent || !aParent->IsHTML()) {
+  if (!aParent || !aParent->IsHTMLElement()) {
     return true;
   }
 
@@ -2539,7 +2540,7 @@ ShouldEscape(nsIContent* aParent)
     }
   }
 
-  nsIAtom* tag = aParent->Tag();
+  nsIAtom* tag = aParent->NodeInfo()->NameAtom();
   if (sFilter.mightContain(tag)) {
     for (uint32_t i = 0; i < ArrayLength(nonEscapingElements); ++i) {
       if (tag == nonEscapingElements[i]) {
@@ -2585,10 +2586,10 @@ IsVoidTag(nsIAtom* aTag)
 static inline bool
 IsVoidTag(Element* aElement)
 {
-  if (!aElement->IsHTML()) {
+  if (!aElement->IsHTMLElement()) {
     return false;
   }
-  return IsVoidTag(aElement->Tag());
+  return IsVoidTag(aElement->NodeInfo()->NameAtom());
 }
 
 /* static */
@@ -2665,8 +2666,9 @@ Serialize(FragmentOrElement* aRoot, bool aDescendentsOnly, nsAString& aOut)
       if (!isVoid && current->NodeType() == nsIDOMNode::ELEMENT_NODE) {
         builder.Append("</");
         nsIContent* elem = static_cast<nsIContent*>(current);
-        if (elem->IsHTML() || elem->IsSVG() || elem->IsMathML()) {
-          builder.Append(elem->Tag());
+        if (elem->IsHTMLElement() || elem->IsSVGElement() ||
+            elem->IsMathMLElement()) {
+          builder.Append(elem->NodeInfo()->NameAtom());
         } else {
           builder.Append(current->NodeName());
         }
@@ -2837,17 +2839,17 @@ FragmentOrElement::SetInnerHTMLInternal(const nsAString& aInnerHTML, ErrorResult
 
   nsAutoScriptLoaderDisabler sld(doc);
 
-  nsIAtom* contextLocalName = Tag();
+  nsIAtom* contextLocalName = NodeInfo()->NameAtom();
   int32_t contextNameSpaceID = GetNameSpaceID();
 
   ShadowRoot* shadowRoot = ShadowRoot::FromNode(this);
   if (shadowRoot) {
     // Fix up the context to be the host of the ShadowRoot.
-    contextLocalName = shadowRoot->GetHost()->Tag();
+    contextLocalName = shadowRoot->GetHost()->NodeInfo()->NameAtom();
     contextNameSpaceID = shadowRoot->GetHost()->GetNameSpaceID();
   }
 
-  if (doc->IsHTML()) {
+  if (doc->IsHTMLDocument()) {
     int32_t oldChildCount = target->GetChildCount();
     aError = nsContentUtils::ParseFragmentHTML(aInnerHTML,
                                                target,
diff --git a/dom/base/Link.cpp b/dom/base/Link.cpp
index 7aeb8ddbbe..e553ef5dd2 100644
--- a/dom/base/Link.cpp
+++ b/dom/base/Link.cpp
@@ -40,8 +40,10 @@ Link::~Link()
 bool
 Link::ElementHasHref() const
 {
-  return ((!mElement->IsSVG() && mElement->HasAttr(kNameSpaceID_None, nsGkAtoms::href))
-        || (!mElement->IsHTML() && mElement->HasAttr(kNameSpaceID_XLink, nsGkAtoms::href)));
+  return ((!mElement->IsSVGElement() &&
+           mElement->HasAttr(kNameSpaceID_None, nsGkAtoms::href))
+        || (!mElement->IsHTMLElement() &&
+            mElement->HasAttr(kNameSpaceID_XLink, nsGkAtoms::href)));
 }
 
 void
diff --git a/dom/base/NodeInfo.cpp b/dom/base/NodeInfo.cpp
index 64fdaeb04e..50ba1e1b4e 100644
--- a/dom/base/NodeInfo.cpp
+++ b/dom/base/NodeInfo.cpp
@@ -83,7 +83,7 @@ NodeInfo::NodeInfo(nsIAtom *aName, nsIAtom *aPrefix, int32_t aNamespaceID,
       // Correct the case for HTML
       if (aNodeType == nsIDOMNode::ELEMENT_NODE &&
           aNamespaceID == kNameSpaceID_XHTML && GetDocument() &&
-          GetDocument()->IsHTML()) {
+          GetDocument()->IsHTMLDocument()) {
         nsContentUtils::ASCIIToUpper(mQualifiedName, mNodeName);
       } else {
         mNodeName = mQualifiedName;
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 1849e0717e..405e6c3e31 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -565,7 +565,7 @@ ShadowRoot::ChangePoolHost(nsIContent* aNewHost)
 bool
 ShadowRoot::IsShadowInsertionPoint(nsIContent* aContent)
 {
-  if (aContent && aContent->IsHTML(nsGkAtoms::shadow)) {
+  if (aContent && aContent->IsHTMLElement(nsGkAtoms::shadow)) {
     HTMLShadowElement* shadowElem = static_cast<HTMLShadowElement*>(aContent);
     return shadowElem->IsInsertionPoint();
   }
@@ -597,7 +597,7 @@ ShadowRoot::IsPooledNode(nsIContent* aContent, nsIContent* aContainer,
     return true;
   }
 
-  if (aContainer && aContainer->IsHTML(nsGkAtoms::content)) {
+  if (aContainer && aContainer->IsHTMLElement(nsGkAtoms::content)) {
     // Fallback content will end up in pool if its parent is a child of the host.
     HTMLContentElement* content = static_cast<HTMLContentElement*>(aContainer);
     return content->IsInsertionPoint() && content->MatchedNodes().IsEmpty() &&
diff --git a/dom/base/nsCCUncollectableMarker.cpp b/dom/base/nsCCUncollectableMarker.cpp
index ead7cd116f..55b78a7f0b 100644
--- a/dom/base/nsCCUncollectableMarker.cpp
+++ b/dom/base/nsCCUncollectableMarker.cpp
@@ -472,7 +472,7 @@ TraceActiveWindowGlobal(const uint64_t& aId, nsGlobalWindow*& aWindow, void* aCl
 
 #ifdef MOZ_XUL
     nsIDocument* doc = aWindow->GetExtantDoc();
-    if (doc && doc->IsXUL()) {
+    if (doc && doc->IsXULDocument()) {
       XULDocument* xulDoc = static_cast<XULDocument*>(doc);
       xulDoc->TraceProtos(closure->mTrc, closure->mGCNumber);
     }
diff --git a/dom/base/nsContentList.cpp b/dom/base/nsContentList.cpp
index a5607bf791..5a6b9abbe5 100644
--- a/dom/base/nsContentList.cpp
+++ b/dom/base/nsContentList.cpp
@@ -420,7 +420,7 @@ nsContentList::nsContentList(nsINode* aRootNode,
   // not parser-created and don't need to be flushing stuff under us
   // to get our kids right.
   nsIDocument* doc = mRootNode->GetUncomposedDoc();
-  mFlushesNeeded = doc && !doc->IsHTML();
+  mFlushesNeeded = doc && !doc->IsHTMLDocument();
 }
 
 nsContentList::nsContentList(nsINode* aRootNode,
@@ -453,7 +453,7 @@ nsContentList::nsContentList(nsINode* aRootNode,
   // not parser-created and don't need to be flushing stuff under us
   // to get our kids right.
   nsIDocument* doc = mRootNode->GetUncomposedDoc();
-  mFlushesNeeded = doc && !doc->IsHTML();
+  mFlushesNeeded = doc && !doc->IsHTMLDocument();
 }
 
 nsContentList::~nsContentList()
@@ -865,7 +865,7 @@ nsContentList::Match(Element *aElement)
     return toReturn;
 
   bool matchHTML = aElement->GetNameSpaceID() == kNameSpaceID_XHTML &&
-    aElement->OwnerDoc()->IsHTML();
+    aElement->OwnerDoc()->IsHTMLDocument();
  
   if (unknown) {
     return matchHTML ? ni->QualifiedNameEquals(mHTMLMatchAtom) :
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 2867960494..548c48d169 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -1526,42 +1526,41 @@ nsContentUtils::IsHTMLWhitespaceOrNBSP(char16_t aChar)
 
 /* static */
 bool
-nsContentUtils::IsHTMLBlock(nsIAtom* aLocalName)
+nsContentUtils::IsHTMLBlock(nsIContent* aContent)
 {
-  return
-    (aLocalName == nsGkAtoms::address) ||
-    (aLocalName == nsGkAtoms::article) ||
-    (aLocalName == nsGkAtoms::aside) ||
-    (aLocalName == nsGkAtoms::blockquote) ||
-    (aLocalName == nsGkAtoms::center) ||
-    (aLocalName == nsGkAtoms::dir) ||
-    (aLocalName == nsGkAtoms::div) ||
-    (aLocalName == nsGkAtoms::dl) || // XXX why not dt and dd?
-    (aLocalName == nsGkAtoms::fieldset) ||
-    (aLocalName == nsGkAtoms::figure) || // XXX shouldn't figcaption be on this list
-    (aLocalName == nsGkAtoms::footer) ||
-    (aLocalName == nsGkAtoms::form) ||
-    (aLocalName == nsGkAtoms::h1) ||
-    (aLocalName == nsGkAtoms::h2) ||
-    (aLocalName == nsGkAtoms::h3) ||
-    (aLocalName == nsGkAtoms::h4) ||
-    (aLocalName == nsGkAtoms::h5) ||
-    (aLocalName == nsGkAtoms::h6) ||
-    (aLocalName == nsGkAtoms::header) ||
-    (aLocalName == nsGkAtoms::hgroup) ||
-    (aLocalName == nsGkAtoms::hr) ||
-    (aLocalName == nsGkAtoms::li) ||
-    (aLocalName == nsGkAtoms::listing) ||
-    (aLocalName == nsGkAtoms::menu) ||
-    (aLocalName == nsGkAtoms::multicol) || // XXX get rid of this one?
-    (aLocalName == nsGkAtoms::nav) ||
-    (aLocalName == nsGkAtoms::ol) ||
-    (aLocalName == nsGkAtoms::p) ||
-    (aLocalName == nsGkAtoms::pre) ||
-    (aLocalName == nsGkAtoms::section) ||
-    (aLocalName == nsGkAtoms::table) ||
-    (aLocalName == nsGkAtoms::ul) ||
-    (aLocalName == nsGkAtoms::xmp);
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::address,
+                                       nsGkAtoms::article,
+                                       nsGkAtoms::aside,
+                                       nsGkAtoms::blockquote,
+                                       nsGkAtoms::center,
+                                       nsGkAtoms::dir,
+                                       nsGkAtoms::div,
+                                       nsGkAtoms::dl, // XXX why not dt and dd?
+                                       nsGkAtoms::fieldset,
+                                       nsGkAtoms::figure, // XXX shouldn't figcaption be on this list
+                                       nsGkAtoms::footer,
+                                       nsGkAtoms::form,
+                                       nsGkAtoms::h1,
+                                       nsGkAtoms::h2,
+                                       nsGkAtoms::h3,
+                                       nsGkAtoms::h4,
+                                       nsGkAtoms::h5,
+                                       nsGkAtoms::h6,
+                                       nsGkAtoms::header,
+                                       nsGkAtoms::hgroup,
+                                       nsGkAtoms::hr,
+                                       nsGkAtoms::li,
+                                       nsGkAtoms::listing,
+                                       nsGkAtoms::menu,
+                                       nsGkAtoms::multicol, // XXX get rid of this one?
+                                       nsGkAtoms::nav,
+                                       nsGkAtoms::ol,
+                                       nsGkAtoms::p,
+                                       nsGkAtoms::pre,
+                                       nsGkAtoms::section,
+                                       nsGkAtoms::table,
+                                       nsGkAtoms::ul,
+                                       nsGkAtoms::xmp);
 }
 
 /* static */
@@ -2564,7 +2563,8 @@ nsContentUtils::GenerateStateKey(nsIContent* aContent,
     // we can't figure out form info.  Append the tag name if it's an element
     // to avoid restoring state for one type of element on another type.
     if (aContent->IsElement()) {
-      KeyAppendString(nsDependentAtomString(aContent->Tag()), aKey);
+      KeyAppendString(nsDependentAtomString(aContent->NodeInfo()->NameAtom()),
+                      aKey);
     }
     else {
       // Append a character that is not "d" or "f" to disambiguate from
@@ -4123,7 +4123,7 @@ nsContentUtils::CreateContextualFragment(nsINode* aContextNode,
   // If we don't have a document here, we can't get the right security context
   // for compiling event handlers... so just bail out.
   nsCOMPtr<nsIDocument> document = aContextNode->OwnerDoc();
-  bool isHTML = document->IsHTML();
+  bool isHTML = document->IsHTMLDocument();
 #ifdef DEBUG
   nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(document);
   NS_ASSERTION(!isHTML || htmlDoc, "Should have HTMLDocument here!");
@@ -4142,9 +4142,9 @@ nsContentUtils::CreateContextualFragment(nsINode* aContextNode,
       }
     }
     
-    if (contextAsContent && !contextAsContent->IsHTML(nsGkAtoms::html)) {
+    if (contextAsContent && !contextAsContent->IsHTMLElement(nsGkAtoms::html)) {
       aRv = ParseFragmentHTML(aFragment, frag,
-                              contextAsContent->Tag(),
+                              contextAsContent->NodeInfo()->NameAtom(),
                               contextAsContent->GetNameSpaceID(),
                               (document->GetCompatibilityMode() ==
                                eCompatibility_NavQuirks),
@@ -4771,8 +4771,9 @@ nsContentUtils::TriggerLink(nsIContent *aContent, nsPresContext *aPresContext,
     // as downloadable. If this check fails we will just do the normal thing
     // (i.e. navigate to the resource).
     nsAutoString fileName;
-    if ((!aContent->IsHTML(nsGkAtoms::a) && !aContent->IsHTML(nsGkAtoms::area) &&
-         !aContent->IsSVG(nsGkAtoms::a)) ||
+    if ((!aContent->IsHTMLElement(nsGkAtoms::a) &&
+         !aContent->IsHTMLElement(nsGkAtoms::area) &&
+         !aContent->IsSVGElement(nsGkAtoms::a)) ||
         !aContent->GetAttr(kNameSpaceID_None, nsGkAtoms::download, fileName) ||
         NS_FAILED(aContent->NodePrincipal()->CheckMayLoad(aLinkURI, false, true))) {
       fileName.SetIsVoid(true); // No actionable download attribute was found.
@@ -6943,7 +6944,7 @@ nsContentUtils::IsContentInsertionPoint(const nsIContent* aContent)
   }
 
   // Check if the content is a web components content insertion point.
-  if (aContent->IsHTML(nsGkAtoms::content)) {
+  if (aContent->IsHTMLElement(nsGkAtoms::content)) {
     return static_cast<const HTMLContentElement*>(aContent)->IsInsertionPoint();
   }
 
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 1bc5ce73fb..29e489233f 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -361,7 +361,7 @@ public:
   /**
    * Is the HTML local name a block element?
    */
-  static bool IsHTMLBlock(nsIAtom* aLocalName);
+  static bool IsHTMLBlock(nsIContent* aContent);
 
   enum ParseHTMLIntegerResultFlags {
     eParseHTMLInteger_NoFlags               = 0,
diff --git a/dom/base/nsCopySupport.cpp b/dom/base/nsCopySupport.cpp
index 9517a30fa6..16a43ef759 100644
--- a/dom/base/nsCopySupport.cpp
+++ b/dom/base/nsCopySupport.cpp
@@ -510,7 +510,7 @@ static nsresult AppendDOMNode(nsITransferable *aTransferable,
   nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(document, &rv);
   NS_ENSURE_SUCCESS(rv, NS_OK);
 
-  NS_ENSURE_TRUE(document->IsHTML(), NS_OK);
+  NS_ENSURE_TRUE(document->IsHTMLDocument(), NS_OK);
 
   // init encoder with document and node
   rv = docEncoder->NativeInit(document, NS_LITERAL_STRING(kHTMLMime),
diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index c3948c12ed..6178965ab7 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -1656,20 +1656,18 @@ nsDOMWindowUtils::GetTranslationNodes(nsIDOMNode* aRoot,
   // skip the root tag from being a translation node.
   nsIContent* content = root;
   while ((limit > 0) && (content = content->GetNextNode(root))) {
-    if (!content->IsHTML()) {
+    if (!content->IsHTMLElement()) {
       continue;
     }
 
-    nsIAtom* localName = content->Tag();
-
     // Skip elements that usually contain non-translatable text content.
-    if (localName == nsGkAtoms::script ||
-        localName == nsGkAtoms::iframe ||
-        localName == nsGkAtoms::frameset ||
-        localName == nsGkAtoms::frame ||
-        localName == nsGkAtoms::code ||
-        localName == nsGkAtoms::noscript ||
-        localName == nsGkAtoms::style) {
+    if (content->IsAnyOfHTMLElements(nsGkAtoms::script,
+                                     nsGkAtoms::iframe,
+                                     nsGkAtoms::frameset,
+                                     nsGkAtoms::frame,
+                                     nsGkAtoms::code,
+                                     nsGkAtoms::noscript,
+                                     nsGkAtoms::style)) {
       continue;
     }
 
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 819b54bf70..3f4557a7cd 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -626,7 +626,7 @@ nsIdentifierMapEntry::RemoveIdElement(Element* aElement)
   // This could fire in OOM situations
   // Only assert this in HTML documents for now as XUL does all sorts of weird
   // crap.
-  NS_ASSERTION(!aElement->OwnerDoc()->IsHTML() ||
+  NS_ASSERTION(!aElement->OwnerDoc()->IsHTMLDocument() ||
                mIdContentList.IndexOf(aElement) >= 0,
                "Removing id entry that doesn't exist");
 
@@ -5473,7 +5473,7 @@ nsIDocument::CreateElement(const nsAString& aTagName, ErrorResult& rv)
     return nullptr;
   }
 
-  bool needsLowercase = IsHTML() && !IsLowercaseASCII(aTagName);
+  bool needsLowercase = IsHTMLDocument() && !IsLowercaseASCII(aTagName);
   nsAutoString lcTagName;
   if (needsLowercase) {
     nsContentUtils::ASCIIToLower(aTagName, lcTagName);
@@ -5497,7 +5497,7 @@ nsDocument::SetupCustomElement(Element* aElement,
     return;
   }
 
-  nsCOMPtr<nsIAtom> tagAtom = aElement->Tag();
+  nsCOMPtr<nsIAtom> tagAtom = aElement->NodeInfo()->NameAtom();
   nsCOMPtr<nsIAtom> typeAtom = aTypeExtension ?
     do_GetAtom(*aTypeExtension) : tagAtom;
 
@@ -5676,7 +5676,7 @@ already_AddRefed<CDATASection>
 nsIDocument::CreateCDATASection(const nsAString& aData,
                                 ErrorResult& rv)
 {
-  if (IsHTML()) {
+  if (IsHTMLDocument()) {
     rv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
     return nullptr;
   }
@@ -6134,7 +6134,7 @@ nsDocument::RegisterElement(JSContext* aCx, const nsAString& aType,
   // Only convert NAME to lowercase in HTML documents. Note that NAME is
   // options.extends.
   nsAutoString lcName;
-  if (IsHTML()) {
+  if (IsHTMLDocument()) {
     nsContentUtils::ASCIIToLower(aOptions.mExtends, lcName);
   } else {
     lcName.Assign(aOptions.mExtends);
@@ -6327,15 +6327,12 @@ nsDocument::RegisterElement(JSContext* aCx, const nsAString& aType,
       // Make sure that the element name matches the name in the definition.
       // (e.g. a definition for x-button extending button should match
       // <button is="x-button"> but not <x-button>.
-      // Note: we also check the tag name, because if it's not the above
-      // mentioned case, it can be that only the |is| property has been
-      // changed, which we should ignore by the spec.
-      if (elem->NodeInfo()->NameAtom() != nameAtom &&
-          elem->Tag() == nameAtom) {
+      if (elem->NodeInfo()->NameAtom() != nameAtom) {
         //Skip over this element because definition does not apply.
         continue;
       }
 
+      MOZ_ASSERT(elem->IsHTMLElement(nameAtom));
       nsWrapperCache* cache;
       CallQueryInterface(elem, &cache);
       MOZ_ASSERT(cache, "Element doesn't support wrapper cache?");
@@ -7002,7 +6999,7 @@ Element*
 nsIDocument::GetHtmlElement() const
 {
   Element* rootElement = GetRootElement();
-  if (rootElement && rootElement->IsHTML(nsGkAtoms::html))
+  if (rootElement && rootElement->IsHTMLElement(nsGkAtoms::html))
     return rootElement;
   return nullptr;
 }
@@ -7019,7 +7016,7 @@ nsIDocument::GetHtmlChildElement(nsIAtom* aTag)
   for (nsIContent* child = html->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(aTag))
+    if (child->IsHTMLElement(aTag))
       return child->AsElement();
   }
   return nullptr;
@@ -7079,7 +7076,7 @@ nsDocument::GetTitle(nsString& aTitle)
       break;
 #endif
     case kNameSpaceID_SVG:
-      if (rootElement->Tag() == nsGkAtoms::svg) {
+      if (rootElement->IsSVGElement(nsGkAtoms::svg)) {
         GetTitleFromElement(kNameSpaceID_SVG, tmp);
         break;
       } // else fall through
@@ -7206,7 +7203,7 @@ nsDocument::GetBoxObjectFor(Element* aElement, ErrorResult& aRv)
     return nullptr;
   }
 
-  if (!mHasWarnedAboutBoxObjects && !aElement->IsXUL()) {
+  if (!mHasWarnedAboutBoxObjects && !aElement->IsXULElement()) {
     mHasWarnedAboutBoxObjects = true;
     nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
                                     NS_LITERAL_CSTRING("BoxObjects"), this,
@@ -8157,7 +8154,7 @@ nsDocument::FlushPendingNotifications(mozFlushType aType)
   // make sure that layout is started as needed.  But we can skip that
   // part if we have no presshell or if it's already done an initial
   // reflow.
-  if ((!IsHTML() ||
+  if ((!IsHTMLDocument() ||
        (aType > Flush_ContentAndNotify && mPresShell &&
         !mPresShell->DidInitialize())) &&
       (mParser || mWeakSink)) {
@@ -8330,7 +8327,7 @@ nsRadioGroupStruct*
 nsDocument::GetRadioGroupInternal(const nsAString& aName) const
 {
 #ifdef DEBUG
-  if (IsHTML()) {
+  if (IsHTMLDocument()) {
     nsAutoString lcName;
     ToLowerCase(aName, lcName);
     MOZ_ASSERT(aName == lcName);
@@ -8349,7 +8346,7 @@ nsRadioGroupStruct*
 nsDocument::GetRadioGroup(const nsAString& aName) const
 {
   nsAutoString tmKey(aName);
-  if (IsHTML()) {
+  if (IsHTMLDocument()) {
     ToLowerCase(tmKey); //should case-insensitive.
   }
 
@@ -8360,7 +8357,7 @@ nsRadioGroupStruct*
 nsDocument::GetOrCreateRadioGroup(const nsAString& aName)
 {
   nsAutoString tmKey(aName);
-  if (IsHTML()) {
+  if (IsHTMLDocument()) {
     ToLowerCase(tmKey); //should case-insensitive.
   }
 
@@ -8430,7 +8427,7 @@ nsDocument::GetNextRadioButton(const nsAString& aName,
     else if (++index >= numRadios) {
       index = 0;
     }
-    NS_ASSERTION(static_cast<nsGenericHTMLFormElement*>(radioGroup->mRadioButtons[index])->IsHTML(nsGkAtoms::input),
+    NS_ASSERTION(static_cast<nsGenericHTMLFormElement*>(radioGroup->mRadioButtons[index])->IsHTMLElement(nsGkAtoms::input),
                  "mRadioButtons holding a non-radio button");
     radio = static_cast<HTMLInputElement*>(radioGroup->mRadioButtons[index]);
   } while (radio->Disabled() && radio != currentRadio);
diff --git a/dom/base/nsDocumentEncoder.cpp b/dom/base/nsDocumentEncoder.cpp
index bcb2ec3564..886dd8dcfb 100644
--- a/dom/base/nsDocumentEncoder.cpp
+++ b/dom/base/nsDocumentEncoder.cpp
@@ -134,8 +134,6 @@ protected:
     return true;
   }
 
-  static bool IsTag(nsIContent* aContent, nsIAtom* aAtom);
-  
   virtual bool IncludeInContext(nsINode *aNode);
 
   nsCOMPtr<nsIDocument>          mDocument;
@@ -530,7 +528,7 @@ nsDocumentEncoder::SerializeToStringIterative(nsINode* aNode,
             current->NodeType() == nsIDOMNode::DOCUMENT_FRAGMENT_NODE) {
           DocumentFragment* frag = static_cast<DocumentFragment*>(current);
           nsIContent* host = frag->GetHost();
-          if (host && host->IsHTML(nsGkAtoms::_template)) {
+          if (host && host->IsHTMLElement(nsGkAtoms::_template)) {
             current = host;
           }
         }
@@ -541,12 +539,6 @@ nsDocumentEncoder::SerializeToStringIterative(nsINode* aNode,
   return NS_OK;
 }
 
-bool 
-nsDocumentEncoder::IsTag(nsIContent* aContent, nsIAtom* aAtom)
-{
-  return aContent && aContent->Tag() == aAtom;
-}
-
 static nsresult
 ConvertAndWrite(const nsAString& aString,
                 nsIOutputStream* aStream,
@@ -1032,7 +1024,7 @@ static bool ParentIsTR(nsIContent* aContent) {
   if (!parent) {
     return false;
   }
-  return parent->IsHTML(nsGkAtoms::tr);
+  return parent->IsHTMLElement(nsGkAtoms::tr);
 }
 
 
@@ -1102,7 +1094,7 @@ nsDocumentEncoder::EncodeToStringWithMaxLength(uint32_t aMaxLength,
           NS_ENSURE_SUCCESS(rv, rv);
         }
         nsCOMPtr<nsIContent> content = do_QueryInterface(node);
-        if (content && content->IsHTML(nsGkAtoms::tr) && !ParentIsTR(content)) {
+        if (content && content->IsHTMLElement(nsGkAtoms::tr) && !ParentIsTR(content)) {
           nsINode* n = content;
           if (!prevNode) {
             // Went from a non-<tr> to a <tr>
@@ -1387,9 +1379,7 @@ nsHTMLCopyEncoder::SetSelection(nsISelection* aSelection)
        selContent = selContent->GetParent())
   {
     // checking for selection inside a plaintext form widget
-    nsIAtom *atom = selContent->Tag();
-    if (atom == nsGkAtoms::input ||
-        atom == nsGkAtoms::textarea)
+    if (selContent->IsAnyOfHTMLElements(nsGkAtoms::input, nsGkAtoms::textarea))
     {
       mIsTextWidget = true;
       break;
@@ -1410,7 +1400,7 @@ nsHTMLCopyEncoder::SetSelection(nsISelection* aSelection)
   
   // also consider ourselves in a text widget if we can't find an html document
   nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(mDocument);
-  if (!(htmlDoc && mDocument->IsHTML())) {
+  if (!(htmlDoc && mDocument->IsHTMLDocument())) {
     mIsTextWidget = true;
     mSelection = aSelection;
     // mMimeType is set to text/plain when encoding starts.
@@ -1523,34 +1513,32 @@ nsHTMLCopyEncoder::IncludeInContext(nsINode *aNode)
   if (!content)
     return false;
 
-  nsIAtom *tag = content->Tag();
-
-  return (tag == nsGkAtoms::b        ||
-          tag == nsGkAtoms::i        ||
-          tag == nsGkAtoms::u        ||
-          tag == nsGkAtoms::a        ||
-          tag == nsGkAtoms::tt       ||
-          tag == nsGkAtoms::s        ||
-          tag == nsGkAtoms::big      ||
-          tag == nsGkAtoms::small    ||
-          tag == nsGkAtoms::strike   ||
-          tag == nsGkAtoms::em       ||
-          tag == nsGkAtoms::strong   ||
-          tag == nsGkAtoms::dfn      ||
-          tag == nsGkAtoms::code     ||
-          tag == nsGkAtoms::cite     ||
-          tag == nsGkAtoms::var      ||
-          tag == nsGkAtoms::abbr     ||
-          tag == nsGkAtoms::font     ||
-          tag == nsGkAtoms::script   ||
-          tag == nsGkAtoms::span     ||
-          tag == nsGkAtoms::pre      ||
-          tag == nsGkAtoms::h1       ||
-          tag == nsGkAtoms::h2       ||
-          tag == nsGkAtoms::h3       ||
-          tag == nsGkAtoms::h4       ||
-          tag == nsGkAtoms::h5       ||
-          tag == nsGkAtoms::h6);
+  return content->IsAnyOfHTMLElements(nsGkAtoms::b,
+                                      nsGkAtoms::i,
+                                      nsGkAtoms::u,
+                                      nsGkAtoms::a,
+                                      nsGkAtoms::tt,
+                                      nsGkAtoms::s,
+                                      nsGkAtoms::big,
+                                      nsGkAtoms::small,
+                                      nsGkAtoms::strike,
+                                      nsGkAtoms::em,
+                                      nsGkAtoms::strong,
+                                      nsGkAtoms::dfn,
+                                      nsGkAtoms::code,
+                                      nsGkAtoms::cite,
+                                      nsGkAtoms::var,
+                                      nsGkAtoms::abbr,
+                                      nsGkAtoms::font,
+                                      nsGkAtoms::script,
+                                      nsGkAtoms::span,
+                                      nsGkAtoms::pre,
+                                      nsGkAtoms::h1,
+                                      nsGkAtoms::h2,
+                                      nsGkAtoms::h3,
+                                      nsGkAtoms::h4,
+                                      nsGkAtoms::h5,
+                                      nsGkAtoms::h6);
 }
 
 
@@ -1714,10 +1702,11 @@ nsHTMLCopyEncoder::GetPromotedPoint(Endpoint aWhere, nsIDOMNode *aNode, int32_t
         if (bResetPromotion)
         {
           nsCOMPtr<nsIContent> content = do_QueryInterface(parent);
-          if (content)
+          if (content && content->IsHTMLElement())
           {
             bool isBlock = false;
-            parserService->IsBlock(parserService->HTMLAtomTagToId(content->Tag()), isBlock);
+            parserService->IsBlock(parserService->HTMLAtomTagToId(
+                                     content->NodeInfo()->NameAtom()), isBlock);
             if (isBlock)
             {
               bResetPromotion = false;
@@ -1796,10 +1785,11 @@ nsHTMLCopyEncoder::GetPromotedPoint(Endpoint aWhere, nsIDOMNode *aNode, int32_t
         if (bResetPromotion)
         {
           nsCOMPtr<nsIContent> content = do_QueryInterface(parent);
-          if (content)
+          if (content && content->IsHTMLElement())
           {
             bool isBlock = false;
-            parserService->IsBlock(parserService->HTMLAtomTagToId(content->Tag()), isBlock);
+            parserService->IsBlock(parserService->HTMLAtomTagToId(
+                                     content->NodeInfo()->NameAtom()), isBlock);
             if (isBlock)
             {
               bResetPromotion = false;
@@ -1858,7 +1848,7 @@ nsHTMLCopyEncoder::IsMozBR(nsIDOMNode* aNode)
   MOZ_ASSERT(aNode);
   nsCOMPtr<Element> element = do_QueryInterface(aNode);
   return element &&
-         element->IsHTML(nsGkAtoms::br) &&
+         element->IsHTMLElement(nsGkAtoms::br) &&
          element->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                               NS_LITERAL_STRING("_moz"), eIgnoreCase);
 }
@@ -1890,16 +1880,17 @@ bool
 nsHTMLCopyEncoder::IsRoot(nsIDOMNode* aNode)
 {
   nsCOMPtr<nsIContent> content = do_QueryInterface(aNode);
-  if (content)
-  {
-    if (mIsTextWidget) 
-      return (IsTag(content, nsGkAtoms::div));
-
-    return (IsTag(content, nsGkAtoms::body) ||
-            IsTag(content, nsGkAtoms::td)   ||
-            IsTag(content, nsGkAtoms::th));
+  if (!content) {
+    return false;
   }
-  return false;
+
+  if (mIsTextWidget) {
+    return content->IsHTMLElement(nsGkAtoms::div);
+  }
+
+  return content->IsAnyOfHTMLElements(nsGkAtoms::body,
+                                      nsGkAtoms::td,
+                                      nsGkAtoms::th);
 }
 
 bool
@@ -2017,11 +2008,12 @@ nsHTMLCopyEncoder::GetImmediateContextCount(const nsTArray<nsINode*>& aAncestorA
       break;
     }
     nsCOMPtr<nsIContent> content(do_QueryInterface(node));
-    if (!content || !content->IsHTML() || (content->Tag() != nsGkAtoms::tr    &&
-                                           content->Tag() != nsGkAtoms::thead &&
-                                           content->Tag() != nsGkAtoms::tbody &&
-                                           content->Tag() != nsGkAtoms::tfoot &&
-                                           content->Tag() != nsGkAtoms::table)) {
+    if (!content ||
+        !content->IsAnyOfHTMLElements(nsGkAtoms::tr,
+                                      nsGkAtoms::thead,
+                                      nsGkAtoms::tbody,
+                                      nsGkAtoms::tfoot,
+                                      nsGkAtoms::table)) {
       break;
     }
     ++j;
diff --git a/dom/base/nsFocusManager.cpp b/dom/base/nsFocusManager.cpp
index c46d7efe7c..5fffe6334b 100644
--- a/dom/base/nsFocusManager.cpp
+++ b/dom/base/nsFocusManager.cpp
@@ -84,7 +84,7 @@ PRLogModuleInfo* gFocusNavigationLog;
   {                                                             \
     nsAutoCString tag(NS_LITERAL_CSTRING("(none)"));            \
     if (content) {                                              \
-      content->Tag()->ToUTF8String(tag);                        \
+      content->NodeInfo()->NameAtom()->ToUTF8String(tag);       \
     }                                                           \
     PR_LOG(log, 4, (format, tag.get()));                        \
   }
@@ -316,7 +316,7 @@ nsIContent*
 nsFocusManager::GetRedirectedFocus(nsIContent* aContent)
 {
 #ifdef MOZ_XUL
-  if (aContent->IsXUL()) {
+  if (aContent->IsXULElement()) {
     nsCOMPtr<nsIDOMNode> inputField;
 
     nsCOMPtr<nsIDOMXULTextBoxElement> textbox = do_QueryInterface(aContent);
@@ -328,7 +328,7 @@ nsFocusManager::GetRedirectedFocus(nsIContent* aContent)
       if (menulist) {
         menulist->GetInputField(getter_AddRefs(inputField));
       }
-      else if (aContent->Tag() == nsGkAtoms::scale) {
+      else if (aContent->IsXULElement(nsGkAtoms::scale)) {
         nsCOMPtr<nsIDocument> doc = aContent->GetComposedDoc();
         if (!doc)
           return nullptr;
@@ -336,7 +336,7 @@ nsFocusManager::GetRedirectedFocus(nsIContent* aContent)
         nsINodeList* children = doc->BindingManager()->GetAnonymousNodesFor(aContent);
         if (children) {
           nsIContent* child = children->Item(0);
-          if (child && child->Tag() == nsGkAtoms::slider)
+          if (child && child->IsXULElement(nsGkAtoms::slider))
             return child;
         }
       }
@@ -1552,7 +1552,7 @@ nsFocusManager::CheckIfFocusable(nsIContent* aContent, uint32_t aFlags)
     return nullptr;
   }
 
-  if (aContent->Tag() == nsGkAtoms::area && aContent->IsHTML()) {
+  if (aContent->IsHTMLElement(nsGkAtoms::area)) {
     // HTML areas do not have their own frame, and the img frame we get from
     // GetPrimaryFrame() is not relevant as to whether it is focusable or
     // not, so we have to do all the relevant checks manually for them.
@@ -2503,8 +2503,7 @@ nsFocusManager::DetermineElementToMoveFocus(nsPIDOMWindow* aWindow,
   int32_t tabIndex = forward ? 1 : 0;
   if (startContent) {
     nsIFrame* frame = startContent->GetPrimaryFrame();
-    if (startContent->Tag() == nsGkAtoms::area &&
-        startContent->IsHTML())
+    if (startContent->IsHTMLElement(nsGkAtoms::area))
       startContent->IsFocusable(&tabIndex);
     else if (frame)
       frame->IsFocusable(&tabIndex, 0);
@@ -2812,8 +2811,8 @@ nsFocusManager::GetNextTabbableContent(nsIPresShell* aPresShell,
       }
     }
     else if (getNextFrame &&
-             (!iterStartContent || iterStartContent->Tag() != nsGkAtoms::area ||
-              !iterStartContent->IsHTML())) {
+             (!iterStartContent ||
+              !iterStartContent->IsHTMLElement(nsGkAtoms::area))) {
       // Need to do special check in case we're in an imagemap which has multiple
       // content nodes per frame, so don't skip over the starting frame.
       if (aForward)
@@ -2842,7 +2841,7 @@ nsFocusManager::GetNextTabbableContent(nsIPresShell* aPresShell,
       nsIContent* currentContent = frame->GetContent();
       if (tabIndex >= 0) {
         NS_ASSERTION(currentContent, "IsFocusable set a tabindex for a frame with no content");
-        if (currentContent->Tag() == nsGkAtoms::img &&
+        if (currentContent->IsHTMLElement(nsGkAtoms::img) &&
             currentContent->HasAttr(kNameSpaceID_None, nsGkAtoms::usemap)) {
           // This is an image with a map. Image map areas are not traversed by
           // nsIFrameTraversal so look for the next or previous area element.
@@ -3075,7 +3074,7 @@ nsFocusManager::GetRootForFocus(nsPIDOMWindow* aWindow,
     nsCOMPtr<Element> docElement = aWindow->GetFrameElementInternal();
     // document navigation skips iframes and frames that are specifically non-focusable
     if (docElement) {
-      if (docElement->Tag() == nsGkAtoms::iframe)
+      if (docElement->NodeInfo()->NameAtom() == nsGkAtoms::iframe)
         return nullptr;
 
       nsIFrame* frame = docElement->GetPrimaryFrame();
@@ -3228,7 +3227,7 @@ nsFocusManager::GetNextTabbablePanel(nsIDocument* aDocument, nsIFrame* aCurrentP
     }
 
     // Skip over non-panels
-    if (popupFrame->GetContent()->Tag() != nsGkAtoms::panel ||
+    if (!popupFrame->GetContent()->IsXULElement(nsGkAtoms::panel) ||
         (aDocument && popupFrame->GetContent()->GetComposedDoc() != aDocument)) {
       continue;
     }
diff --git a/dom/base/nsFrameLoader.cpp b/dom/base/nsFrameLoader.cpp
index b476362ae2..1a349111e0 100644
--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -208,7 +208,7 @@ nsFrameLoader::LoadFrame()
 
   nsAutoString src;
 
-  bool isSrcdoc = mOwnerContent->IsHTML(nsGkAtoms::iframe) &&
+  bool isSrcdoc = mOwnerContent->IsHTMLElement(nsGkAtoms::iframe) &&
                   mOwnerContent->HasAttr(kNameSpaceID_None, nsGkAtoms::srcdoc);
   if (isSrcdoc) {
     src.AssignLiteral("about:srcdoc");
@@ -222,7 +222,7 @@ nsFrameLoader::LoadFrame()
       // If the frame is a XUL element and has the attribute 'nodefaultsrc=true'
       // then we will not use 'about:blank' as fallback but return early without
       // starting a load if no 'src' attribute is given (or it's empty).
-      if (mOwnerContent->IsXUL() &&
+      if (mOwnerContent->IsXULElement() &&
           mOwnerContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::nodefaultsrc,
                                      nsGkAtoms::_true, eCaseMatters)) {
         return NS_OK;
@@ -402,7 +402,7 @@ nsFrameLoader::ReallyStartLoadingInternal()
   nsCOMPtr<nsIURI> referrer;
   
   nsAutoString srcdoc;
-  bool isSrcdoc = mOwnerContent->IsHTML(nsGkAtoms::iframe) &&
+  bool isSrcdoc = mOwnerContent->IsHTMLElement(nsGkAtoms::iframe) &&
                   mOwnerContent->GetAttr(kNameSpaceID_None, nsGkAtoms::srcdoc,
                                          srcdoc);
 
@@ -1729,7 +1729,7 @@ nsFrameLoader::MaybeCreateDocShell()
   }
 
   if (mIsTopLevelContent &&
-      mOwnerContent->IsXUL(nsGkAtoms::browser) &&
+      mOwnerContent->IsXULElement(nsGkAtoms::browser) &&
       !mOwnerContent->HasAttr(kNameSpaceID_None, nsGkAtoms::disablehistory)) {
     nsresult rv;
     nsCOMPtr<nsISHistory> sessionHistory =
@@ -1815,7 +1815,7 @@ nsFrameLoader::GetURL(nsString& aURI)
 {
   aURI.Truncate();
 
-  if (mOwnerContent->Tag() == nsGkAtoms::object) {
+  if (mOwnerContent->IsHTMLElement(nsGkAtoms::object)) {
     mOwnerContent->GetAttr(kNameSpaceID_None, nsGkAtoms::data, aURI);
   } else {
     mOwnerContent->GetAttr(kNameSpaceID_None, nsGkAtoms::src, aURI);
@@ -2115,7 +2115,7 @@ nsFrameLoader::TryRemoteBrowser()
       return false;
     }
 
-    if (!mOwnerContent->IsXUL()) {
+    if (!mOwnerContent->IsXULElement()) {
       return false;
     }
 
@@ -2424,7 +2424,7 @@ nsFrameLoader::EnsureMessageManager()
   if (!mIsTopLevelContent &&
       !OwnerIsBrowserOrAppFrame() &&
       !mRemoteFrame &&
-      !(mOwnerContent->IsXUL() &&
+      !(mOwnerContent->IsXULElement() &&
         mOwnerContent->AttrValueIs(kNameSpaceID_None,
                                    nsGkAtoms::forcemessagemanager,
                                    nsGkAtoms::_true, eCaseMatters))) {
@@ -2445,7 +2445,7 @@ nsFrameLoader::EnsureMessageManager()
 
   if (chromeWindow) {
     nsAutoString messagemanagergroup;
-    if (mOwnerContent->IsXUL() &&
+    if (mOwnerContent->IsXULElement() &&
         mOwnerContent->GetAttr(kNameSpaceID_None,
                                nsGkAtoms::messagemanagergroup,
                                messagemanagergroup)) {
diff --git a/dom/base/nsFrameLoader.h b/dom/base/nsFrameLoader.h
index 07df0d505f..8531ddaa5c 100644
--- a/dom/base/nsFrameLoader.h
+++ b/dom/base/nsFrameLoader.h
@@ -306,7 +306,8 @@ private:
                               nsIDocShell* aParentNode);
 
   nsIAtom* TypeAttrName() const {
-    return mOwnerContent->IsXUL() ? nsGkAtoms::type : nsGkAtoms::mozframetype;
+    return mOwnerContent->IsXULElement()
+             ? nsGkAtoms::type : nsGkAtoms::mozframetype;
   }
 
   // Update the permission manager's app-id refcount based on mOwnerContent's
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 7dd894a746..8bb629c081 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -10086,7 +10086,7 @@ nsGlobalWindow::SetChromeEventHandler(EventTarget* aChromeEventHandler)
 
 static bool IsLink(nsIContent* aContent)
 {
-  return aContent && (aContent->IsHTML(nsGkAtoms::a) ||
+  return aContent && (aContent->IsHTMLElement(nsGkAtoms::a) ||
                       aContent->AttrValueIs(kNameSpaceID_XLink, nsGkAtoms::type,
                                             nsGkAtoms::simple, eCaseMatters));
 }
diff --git a/dom/base/nsHTMLContentSerializer.cpp b/dom/base/nsHTMLContentSerializer.cpp
index 3b226ae4e0..65cd9e7623 100644
--- a/dom/base/nsHTMLContentSerializer.cpp
+++ b/dom/base/nsHTMLContentSerializer.cpp
@@ -193,7 +193,7 @@ nsHTMLContentSerializer::AppendElementStart(Element* aElement,
 
   NS_ENSURE_SUCCESS(rv, rv);
 
-  nsIAtom *name = content->Tag();
+  nsIAtom *name = content->NodeInfo()->NameAtom();
   int32_t ns = content->GetNameSpaceID();
 
   bool lineBreakBeforeOpen = LineBreakBeforeOpen(ns, name);
@@ -306,7 +306,7 @@ nsHTMLContentSerializer::AppendElementEnd(Element* aElement,
 
   nsIContent* content = aElement;
 
-  nsIAtom *name = content->Tag();
+  nsIAtom *name = content->NodeInfo()->NameAtom();
   int32_t ns = content->GetNameSpaceID();
 
   if (ns == kNameSpaceID_XHTML &&
diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index fa404982e3..46af5eac05 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -39,8 +39,8 @@ enum nsLinkState {
 
 // IID for the nsIContent interface
 #define NS_ICONTENT_IID \
-{ 0x697a2fe1, 0x5549, 0x48e7, \
-  { 0x9a, 0x1a, 0xc2, 0x9d, 0xab, 0x14, 0xe2, 0x39 } }
+{ 0x70f7e9ea, 0xa9bf, 0x48cc, \
+  { 0xad, 0x9d, 0x8a, 0xca, 0xee, 0xd2, 0x9b, 0x68 } }
 
 /**
  * A node of content in a document's content model. This interface
@@ -250,60 +250,69 @@ public:
     return mNodeInfo->NamespaceID();
   }
 
-  /**
-   * Get the NodeInfo for this element
-   * @return the nodes node info
-   */
-  inline mozilla::dom::NodeInfo* NodeInfo() const
-  {
-    return mNodeInfo;
-  }
-
-  inline bool IsInNamespace(int32_t aNamespace) const
-  {
-    return mNodeInfo->NamespaceID() == aNamespace;
-  }
-
-  inline bool IsHTML() const
+  inline bool IsHTMLElement() const
   {
     return IsInNamespace(kNameSpaceID_XHTML);
   }
 
-  inline bool IsHTML(nsIAtom* aTag) const
+  inline bool IsHTMLElement(nsIAtom* aTag) const
   {
     return mNodeInfo->Equals(aTag, kNameSpaceID_XHTML);
   }
 
-  inline bool IsSVG() const
+  template<typename First, typename... Args>
+  inline bool IsAnyOfHTMLElements(First aFirst, Args... aArgs) const
+  {
+    return IsHTMLElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsSVGElement() const
   {
     return IsInNamespace(kNameSpaceID_SVG);
   }
 
-  inline bool IsSVG(nsIAtom* aTag) const
+  inline bool IsSVGElement(nsIAtom* aTag) const
   {
     return mNodeInfo->Equals(aTag, kNameSpaceID_SVG);
   }
 
-  inline bool IsXUL() const
+  template<typename First, typename... Args>
+  inline bool IsAnyOfSVGElements(First aFirst, Args... aArgs) const
+  {
+    return IsSVGElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsXULElement() const
   {
     return IsInNamespace(kNameSpaceID_XUL);
   }
 
-  inline bool IsXUL(nsIAtom* aTag) const
+  inline bool IsXULElement(nsIAtom* aTag) const
   {
     return mNodeInfo->Equals(aTag, kNameSpaceID_XUL);
   }
 
-  inline bool IsMathML() const
+  template<typename First, typename... Args>
+  inline bool IsAnyOfXULElements(First aFirst, Args... aArgs) const
+  {
+    return IsXULElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsMathMLElement() const
   {
     return IsInNamespace(kNameSpaceID_MathML);
   }
 
-  inline bool IsMathML(nsIAtom* aTag) const
+  inline bool IsMathMLElement(nsIAtom* aTag) const
   {
     return mNodeInfo->Equals(aTag, kNameSpaceID_MathML);
   }
 
+  template<typename First, typename... Args>
+  inline bool IsAnyOfMathMLElements(First aFirst, Args... aArgs) const
+  {
+    return IsMathMLElement() && IsNodeInternal(aFirst, aArgs...);
+  }
   inline bool IsActiveChildrenElement() const
   {
     return mNodeInfo->Equals(nsGkAtoms::children, kNameSpaceID_XBL) &&
@@ -919,7 +928,7 @@ public:
         // XHTML1 section C.7).
         bool hasAttr = content->GetAttr(kNameSpaceID_XML, nsGkAtoms::lang,
                                           aResult);
-        if (!hasAttr && (content->IsHTML() || content->IsSVG())) {
+        if (!hasAttr && (content->IsHTMLElement() || content->IsSVGElement())) {
           hasAttr = content->GetAttr(kNameSpaceID_None, nsGkAtoms::lang,
                                      aResult);
         }
diff --git a/dom/base/nsIContentInlines.h b/dom/base/nsIContentInlines.h
index 845c5e9d8a..3782a1818a 100644
--- a/dom/base/nsIContentInlines.h
+++ b/dom/base/nsIContentInlines.h
@@ -15,7 +15,7 @@
 inline bool
 nsIContent::IsInHTMLDocument() const
 {
-  return OwnerDoc()->IsHTML();
+  return OwnerDoc()->IsHTMLDocument();
 }
 
 inline void
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 10fec89561..c0140e8f42 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -149,10 +149,9 @@ struct FullScreenOptions {
 } // namespace dom
 } // namespace mozilla
 
-// 137c6144-513e-4edf-840e-5e3156638ed6
 #define NS_IDOCUMENT_IID \
-{ 0x137c6144, 0x513e, 0x4edf, \
-  { 0x84, 0x0e, 0x5e, 0x31, 0x56, 0x63, 0x8e, 0xd6 } }
+{ 0x0b78eabe, 0x8b94, 0x4ea1, \
+  { 0x93, 0x31, 0x5d, 0x48, 0xe8, 0x3a, 0xda, 0x95 } }
 
 // Enum for requesting a particular type of document when creating a doc
 enum DocumentFlavor {
@@ -1325,7 +1324,7 @@ public:
    * media documents).  Returns false for XHTML and any other documents parsed
    * by the XML parser.
    */
-  bool IsHTML() const
+  bool IsHTMLDocument() const
   {
     return mType == eHTML;
   }
@@ -1333,15 +1332,15 @@ public:
   {
     return mType == eHTML || mType == eXHTML;
   }
-  bool IsXML() const
+  bool IsXMLDocument() const
   {
-    return !IsHTML();
+    return !IsHTMLDocument();
   }
-  bool IsSVG() const
+  bool IsSVGDocument() const
   {
     return mType == eSVG;
   }
-  bool IsXUL() const
+  bool IsXULDocument() const
   {
     return mType == eXUL;
   }
@@ -1351,7 +1350,7 @@ public:
   }
   bool LoadsFullXULStyleSheetUpFront()
   {
-    return IsXUL() || AllowXULXBL();
+    return IsXULDocument() || AllowXULXBL();
   }
 
   virtual bool IsScriptEnabled() = 0;
diff --git a/dom/base/nsINode.cpp b/dom/base/nsINode.cpp
index 956c09a2ce..013b4b9ec6 100644
--- a/dom/base/nsINode.cpp
+++ b/dom/base/nsINode.cpp
@@ -220,7 +220,7 @@ nsINode::GetTextEditorRootContent(nsIEditor** aEditor)
     *aEditor = nullptr;
   for (nsINode* node = this; node; node = node->GetParentNode()) {
     if (!node->IsElement() ||
-        !node->AsElement()->IsHTML())
+        !node->IsHTMLElement())
       continue;
 
     nsCOMPtr<nsIEditor> editor =
@@ -427,7 +427,7 @@ nsINode::IsAnonymousContentInSVGUseSubtree() const
   MOZ_ASSERT(IsInAnonymousSubtree());
   nsIContent* parent = AsContent()->GetBindingParent();
   // Watch out for parentless native-anonymous subtrees.
-  return parent && parent->IsSVG(nsGkAtoms::use);
+  return parent && parent->IsSVGElement(nsGkAtoms::use);
 }
 
 nsresult
@@ -1905,7 +1905,7 @@ bool IsAllowedAsChild(nsIContent* aNewChild, nsINode* aParent,
         // HTML template elements and ShadowRoot hosts need
         // to be checked to ensure that they are not inserted into
         // the hosted content.
-        aNewChild->Tag() == nsGkAtoms::_template ||
+        aNewChild->NodeInfo()->NameAtom() == nsGkAtoms::_template ||
         aNewChild->GetShadowRoot()) &&
        nsContentUtils::ContentIsHostIncludingDescendantOf(aParent,
                                                           aNewChild))) {
diff --git a/dom/base/nsINode.h b/dom/base/nsINode.h
index cbe946b4ad..ab0525c054 100644
--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -250,8 +250,8 @@ private:
 
 // IID for the nsINode interface
 #define NS_INODE_IID \
-{ 0x66972940, 0x1d1b, 0x4d15, \
- { 0x93, 0x11, 0x96, 0x72, 0x84, 0x2e, 0xc7, 0x27 } }
+{ 0xe8fdd227, 0x27da, 0x46ee, \
+  { 0xbe, 0xf3, 0x1a, 0xef, 0x5a, 0x8f, 0xc5, 0xb4 } }
 
 /**
  * An internal interface that abstracts some DOMNode-related parts that both
@@ -570,14 +570,96 @@ public:
   }
 
   /**
-   * Get the tag for this element. This will always return a non-null atom
-   * pointer (as implied by the naming of the method).  For elements this is
-   * the non-namespaced tag, and for other nodes it's something like "#text",
-   * "#comment", "#document", etc.
+   * Get the NodeInfo for this element
+   * @return the nodes node info
    */
-  nsIAtom* Tag() const
+  inline mozilla::dom::NodeInfo* NodeInfo() const
   {
-    return mNodeInfo->NameAtom();
+    return mNodeInfo;
+  }
+
+  inline bool IsInNamespace(int32_t aNamespace) const
+  {
+    return mNodeInfo->NamespaceID() == aNamespace;
+  }
+
+protected:
+  // These 2 methods are useful for the recursive templates IsHTMLElement,
+  // IsSVGElement, etc.
+  inline bool IsNodeInternal() const
+  {
+    return false;
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsNodeInternal(First aFirst, Args... aArgs) const
+  {
+    return mNodeInfo->Equals(aFirst) || IsNodeInternal(aArgs...);
+  }
+
+public:
+  inline bool IsHTMLElement() const
+  {
+    return IsElement() && IsInNamespace(kNameSpaceID_XHTML);
+  }
+
+  inline bool IsHTMLElement(nsIAtom* aTag) const
+  {
+    return IsElement() && mNodeInfo->Equals(aTag, kNameSpaceID_XHTML);
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsAnyOfHTMLElements(First aFirst, Args... aArgs) const
+  {
+    return IsHTMLElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsSVGElement() const
+  {
+    return IsElement() && IsInNamespace(kNameSpaceID_SVG);
+  }
+
+  inline bool IsSVGElement(nsIAtom* aTag) const
+  {
+    return IsElement() && mNodeInfo->Equals(aTag, kNameSpaceID_SVG);
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsAnyOfSVGElements(First aFirst, Args... aArgs) const
+  {
+    return IsSVGElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsXULElement() const
+  {
+    return IsElement() && IsInNamespace(kNameSpaceID_XUL);
+  }
+
+  inline bool IsXULElement(nsIAtom* aTag) const
+  {
+    return IsElement() && mNodeInfo->Equals(aTag, kNameSpaceID_XUL);
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsAnyOfXULElements(First aFirst, Args... aArgs) const
+  {
+    return IsXULElement() && IsNodeInternal(aFirst, aArgs...);
+  }
+
+  inline bool IsMathMLElement() const
+  {
+    return IsElement() && IsInNamespace(kNameSpaceID_MathML);
+  }
+
+  inline bool IsMathMLElement(nsIAtom* aTag) const
+  {
+    return IsElement() && mNodeInfo->Equals(aTag, kNameSpaceID_MathML);
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsAnyOfMathMLElements(First aFirst, Args... aArgs) const
+  {
+    return IsMathMLElement() && IsNodeInternal(aFirst, aArgs...);
   }
 
   /**
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 9052e50a65..89b64ad6f7 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -224,7 +224,7 @@ nsNodeUtils::LastRelease(nsINode* aNode)
       static_cast<nsGenericHTMLFormElement*>(aNode)->ClearForm(true);
     }
 
-    if (aNode->IsElement() && aNode->AsElement()->IsHTML(nsGkAtoms::img) &&
+    if (aNode->IsHTMLElement(nsGkAtoms::img) &&
         aNode->HasFlag(ADDED_TO_FORM)) {
       HTMLImageElement* imageElem = static_cast<HTMLImageElement*>(aNode);
       imageElem->ClearForm(true);
@@ -542,8 +542,7 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
   // cloning, so kids of the new node aren't confused about whether they're
   // in a document.
 #ifdef MOZ_XUL
-  if (aClone && !aParent && aNode->IsElement() &&
-      aNode->AsElement()->IsXUL()) {
+  if (aClone && !aParent && aNode->IsXULElement()) {
     if (!aNode->OwnerDoc()->IsLoadedAsInteractiveData()) {
       clone->SetFlags(NODE_FORCE_XBL_BINDINGS);
     }
@@ -571,7 +570,7 @@ nsNodeUtils::UnlinkUserData(nsINode *aNode)
 bool
 nsNodeUtils::IsTemplateElement(const nsINode *aNode)
 {
-  return aNode->IsElement() && aNode->AsElement()->IsHTML(nsGkAtoms::_template);
+  return aNode->IsHTMLElement(nsGkAtoms::_template);
 }
 
 nsIContent*
diff --git a/dom/base/nsObjectLoadingContent.cpp b/dom/base/nsObjectLoadingContent.cpp
index fc2b04e47d..56aa504494 100644
--- a/dom/base/nsObjectLoadingContent.cpp
+++ b/dom/base/nsObjectLoadingContent.cpp
@@ -994,7 +994,7 @@ nsObjectLoadingContent::BuildParametersArray()
 
   int32_t start = 0, end = content->GetAttrCount(), step = 1;
   // HTML attributes are stored in reverse order.
-  if (content->IsHTML() && content->IsInHTMLDocument()) {
+  if (content->IsHTMLElement() && content->IsInHTMLDocument()) {
     start = end - 1;
     end = -1;
     step = -1;
@@ -1046,7 +1046,7 @@ nsObjectLoadingContent::BuildParametersArray()
   // Nav 4.x would simply replace the "data" with "src". Because some plugins correctly
   // look for "data", lets instead copy the "data" attribute and add another entry
   // to the bottom of the array if there isn't already a "src" specified.
-  if (content->Tag() == nsGkAtoms::object &&
+  if (content->IsHTMLElement(nsGkAtoms::object) &&
       !content->HasAttr(kNameSpaceID_None, nsGkAtoms::src)) {
     MozPluginParameter param;
     content->GetAttr(kNameSpaceID_None, nsGkAtoms::data, param.mValue);
@@ -2930,13 +2930,13 @@ nsObjectLoadingContent::LoadFallback(FallbackType aType, bool aNotify) {
   do_QueryInterface(static_cast<nsIImageLoadingContent*>(this));
   NS_ASSERTION(thisContent, "must be a content");
 
-  if (!thisContent->IsHTML() || mContentType.IsEmpty()) {
+  if (!thisContent->IsHTMLElement() || mContentType.IsEmpty()) {
     // Don't let custom fallback handlers run outside HTML, tags without a
     // determined type should always just be alternate content
     aType = eFallbackAlternate;
   }
 
-  if (thisContent->Tag() == nsGkAtoms::object &&
+  if (thisContent->IsHTMLElement(nsGkAtoms::object) &&
       (aType == eFallbackUnsupported ||
        aType == eFallbackDisabled ||
        aType == eFallbackBlocklisted))
@@ -2944,7 +2944,7 @@ nsObjectLoadingContent::LoadFallback(FallbackType aType, bool aNotify) {
     // Show alternate content instead, if it exists
     for (nsIContent* child = thisContent->GetFirstChild();
          child; child = child->GetNextSibling()) {
-      if (!child->IsHTML(nsGkAtoms::param) &&
+      if (!child->IsHTMLElement(nsGkAtoms::param) &&
           nsStyleUtil::IsSignificantChild(child, true, false)) {
         aType = eFallbackAlternate;
         break;
diff --git a/dom/base/nsPlainTextSerializer.cpp b/dom/base/nsPlainTextSerializer.cpp
index 31dcdc199b..875520ec27 100644
--- a/dom/base/nsPlainTextSerializer.cpp
+++ b/dom/base/nsPlainTextSerializer.cpp
@@ -431,7 +431,7 @@ nsPlainTextSerializer::DoOpenContainer(nsIAtom* aTag)
 {
   // Check if we need output current node as placeholder character and ignore
   // child nodes.
-  if (ShouldReplaceContainerWithPlaceholder(mElement->Tag())) {
+  if (ShouldReplaceContainerWithPlaceholder(mElement->NodeInfo()->NameAtom())) {
     if (mIgnoredChildNodeLevel == 0) {
       // Serialize current node as placeholder character
       Write(NS_LITERAL_STRING("\xFFFC"));
@@ -776,7 +776,7 @@ nsPlainTextSerializer::DoOpenContainer(nsIAtom* aTag)
 nsresult
 nsPlainTextSerializer::DoCloseContainer(nsIAtom* aTag)
 {
-  if (ShouldReplaceContainerWithPlaceholder(mElement->Tag())) {
+  if (ShouldReplaceContainerWithPlaceholder(mElement->NodeInfo()->NameAtom())) {
     mIgnoredChildNodeLevel--;
     return NS_OK;
   }
@@ -1773,11 +1773,11 @@ nsPlainTextSerializer::IsCurrentNodeConverted()
 nsIAtom*
 nsPlainTextSerializer::GetIdForContent(nsIContent* aContent)
 {
-  if (!aContent->IsHTML()) {
+  if (!aContent->IsHTMLElement()) {
     return nullptr;
   }
 
-  nsIAtom* localName = aContent->Tag();
+  nsIAtom* localName = aContent->NodeInfo()->NameAtom();
   return localName->IsStaticAtom() ? localName : nullptr;
 }
 
@@ -1812,7 +1812,7 @@ nsPlainTextSerializer::IsElementBlock(Element* aElement)
     return displayStyle->IsBlockOutsideStyle();
   }
   // Fall back to looking at the tag, in case there is no style information.
-  return nsContentUtils::IsHTMLBlock(GetIdForContent(aElement));
+  return nsContentUtils::IsHTMLBlock(aElement);
 }
 
 /**
diff --git a/dom/base/nsRange.cpp b/dom/base/nsRange.cpp
index 76f8f3eea5..3687c73b4c 100644
--- a/dom/base/nsRange.cpp
+++ b/dom/base/nsRange.cpp
@@ -1118,11 +1118,12 @@ nsRange::IsValidBoundary(nsINode* aNode)
   }
 
   if (aNode->IsNodeOfType(nsINode::eCONTENT)) {
-    nsIContent* content = static_cast<nsIContent*>(aNode);
-    if (content->Tag() == nsGkAtoms::documentTypeNodeName) {
+    if (aNode->NodeInfo()->NameAtom() == nsGkAtoms::documentTypeNodeName) {
       return nullptr;
     }
 
+    nsIContent* content = static_cast<nsIContent*>(aNode);
+
     if (!mMaySpanAnonymousSubtrees) {
       // If the node is in a shadow tree then the ShadowRoot is the root.
       ShadowRoot* containingShadow = content->GetContainingShadow();
@@ -3244,7 +3245,7 @@ IsVisibleAndNotInReplacedElement(nsIFrame* aFrame)
   }
   for (nsIFrame* f = aFrame->GetParent(); f; f = f->GetParent()) {
     if (f->IsFrameOfType(nsIFrame::eReplaced) &&
-        !f->GetContent()->IsHTML(nsGkAtoms::button)) {
+        !f->GetContent()->IsHTMLElement(nsGkAtoms::button)) {
       return false;
     }
   }
@@ -3291,7 +3292,7 @@ enum TreeTraversalState {
 static int8_t
 GetRequiredInnerTextLineBreakCount(nsIFrame* aFrame)
 {
-  if (aFrame->GetContent()->IsHTML(nsGkAtoms::p)) {
+  if (aFrame->GetContent()->IsHTMLElement(nsGkAtoms::p)) {
     return 2;
   }
   const nsStyleDisplay* styleDisplay = aFrame->StyleDisplay();
@@ -3386,7 +3387,7 @@ nsRange::GetInnerTextNoFlush(DOMString& aValue, ErrorResult& aError,
     bool isVisibleAndNotReplaced = IsVisibleAndNotInReplacedElement(f);
     if (currentState == AT_NODE) {
       bool isText = currentNode->IsNodeOfType(nsINode::eTEXT);
-      if (isText && currentNode->GetParent()->IsHTML(nsGkAtoms::rp) &&
+      if (isText && currentNode->GetParent()->IsHTMLElement(nsGkAtoms::rp) &&
           ElementIsVisibleNoFlush(currentNode->GetParent()->AsElement())) {
         nsAutoString str;
         currentNode->GetTextContent(str, aError);
@@ -3409,7 +3410,7 @@ nsRange::GetInnerTextNoFlush(DOMString& aValue, ErrorResult& aError,
       break;
     }
     if (isVisibleAndNotReplaced) {
-      if (currentNode->IsHTML(nsGkAtoms::br)) {
+      if (currentNode->IsHTMLElement(nsGkAtoms::br)) {
         result.Append('\n');
       }
       switch (f->StyleDisplay()->mDisplay) {
@@ -3442,4 +3443,4 @@ nsRange::GetInnerTextNoFlush(DOMString& aValue, ErrorResult& aError,
   }
   // Do not flush trailing line breaks! Required breaks at the end of the text
   // are suppressed.
-}
\ No newline at end of file
+}
diff --git a/dom/base/nsScriptLoader.cpp b/dom/base/nsScriptLoader.cpp
index 72a7c6d766..5894b32efd 100644
--- a/dom/base/nsScriptLoader.cpp
+++ b/dom/base/nsScriptLoader.cpp
@@ -189,7 +189,7 @@ NS_IMPL_ISUPPORTS(nsScriptLoader, nsIStreamLoaderObserver)
 static bool
 IsScriptEventHandler(nsIContent* aScriptElement)
 {
-  if (!aScriptElement->IsHTML()) {
+  if (!aScriptElement->IsHTMLElement()) {
     return false;
   }
 
@@ -595,7 +595,7 @@ nsScriptLoader::ProcessScriptElement(nsIScriptElement *aElement)
     // no 'type=' element
     // "language" is a deprecated attribute of HTML, so we check it only for
     // HTML script elements.
-    if (scriptContent->IsHTML()) {
+    if (scriptContent->IsHTMLElement()) {
       nsAutoString language;
       scriptContent->GetAttr(kNameSpaceID_None, nsGkAtoms::language, language);
       if (!language.IsEmpty()) {
diff --git a/dom/base/nsStyleLinkElement.cpp b/dom/base/nsStyleLinkElement.cpp
index 7cf7ab4559..32e9058c34 100644
--- a/dom/base/nsStyleLinkElement.cpp
+++ b/dom/base/nsStyleLinkElement.cpp
@@ -235,8 +235,8 @@ IsScopedStyleElement(nsIContent* aContent)
   // This is quicker than, say, QIing aContent to nsStyleLinkElement
   // and then calling its virtual GetStyleSheetInfo method to find out
   // if it is scoped.
-  return (aContent->IsHTML(nsGkAtoms::style) ||
-          aContent->IsSVG(nsGkAtoms::style)) &&
+  return (aContent->IsHTMLElement(nsGkAtoms::style) ||
+          aContent->IsSVGElement(nsGkAtoms::style)) &&
          aContent->HasAttr(kNameSpaceID_None, nsGkAtoms::scoped);
 }
 
@@ -315,7 +315,7 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
   // Check for a ShadowRoot because link elements are inert in a
   // ShadowRoot.
   ShadowRoot* containingShadow = thisContent->GetContainingShadow();
-  if (thisContent->IsHTML(nsGkAtoms::link) &&
+  if (thisContent->IsHTMLElement(nsGkAtoms::link) &&
       (aOldShadowRoot || containingShadow)) {
     return NS_OK;
   }
@@ -413,7 +413,7 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
       return NS_ERROR_OUT_OF_MEMORY;
     }
 
-    MOZ_ASSERT(thisContent->Tag() != nsGkAtoms::link,
+    MOZ_ASSERT(thisContent->NodeInfo()->NameAtom() != nsGkAtoms::link,
                "<link> is not 'inline', and needs different CSP checks");
     if (!nsStyleUtil::CSPAllowsInlineStyle(thisContent,
                                            thisContent->NodePrincipal(),
diff --git a/dom/base/nsTreeSanitizer.cpp b/dom/base/nsTreeSanitizer.cpp
index f407585a7a..69747723a5 100644
--- a/dom/base/nsTreeSanitizer.cpp
+++ b/dom/base/nsTreeSanitizer.cpp
@@ -1214,10 +1214,10 @@ nsTreeSanitizer::SanitizeAttributes(mozilla::dom::Element* aElement,
       }
       if (aAllowed->GetEntry(attrLocal) &&
           !((attrLocal == nsGkAtoms::rel &&
-             aElement->IsHTML(nsGkAtoms::link)) ||
+             aElement->IsHTMLElement(nsGkAtoms::link)) ||
             (!mFullDocument &&
              attrLocal == nsGkAtoms::name &&
-             aElement->IsHTML(nsGkAtoms::meta)))) {
+             aElement->IsHTMLElement(nsGkAtoms::meta)))) {
         // name="" and rel="" are whitelisted, but treat them as blacklisted
         // for <meta name> (fragment case) and <link rel> (all cases) to avoid
         // document-wide metadata or styling overrides with non-conforming
@@ -1273,8 +1273,7 @@ nsTreeSanitizer::SanitizeAttributes(mozilla::dom::Element* aElement,
 
   // If we've got HTML audio or video, add the controls attribute, because
   // otherwise the content is unplayable with scripts removed.
-  if (aElement->IsHTML(nsGkAtoms::video) ||
-      aElement->IsHTML(nsGkAtoms::audio)) {
+  if (aElement->IsAnyOfHTMLElements(nsGkAtoms::video, nsGkAtoms::audio)) {
     aElement->SetAttr(kNameSpaceID_None,
                       nsGkAtoms::controls,
                       EmptyString(),
@@ -1358,7 +1357,7 @@ nsTreeSanitizer::Sanitize(nsIDocument* aDocument)
 #ifdef DEBUG
   NS_PRECONDITION(!aDocument->GetContainer(), "The document is in a shell.");
   nsRefPtr<mozilla::dom::Element> root = aDocument->GetRootElement();
-  NS_PRECONDITION(root->IsHTML(nsGkAtoms::html), "Not HTML root.");
+  NS_PRECONDITION(root->IsHTMLElement(nsGkAtoms::html), "Not HTML root.");
 #endif
 
   mFullDocument = true;
diff --git a/dom/base/nsXHTMLContentSerializer.cpp b/dom/base/nsXHTMLContentSerializer.cpp
index 392d5c339f..793393e267 100644
--- a/dom/base/nsXHTMLContentSerializer.cpp
+++ b/dom/base/nsXHTMLContentSerializer.cpp
@@ -459,10 +459,8 @@ nsXHTMLContentSerializer::AfterElementStart(nsIContent* aContent,
                                             nsIContent* aOriginalElement,
                                             nsAString& aStr)
 {
-  nsIAtom *name = aContent->Tag();
-  if (aContent->GetNameSpaceID() == kNameSpaceID_XHTML &&
-      mRewriteEncodingDeclaration &&
-      name == nsGkAtoms::head) {
+  if (mRewriteEncodingDeclaration &&
+      aContent->IsHTMLElement(nsGkAtoms::head)) {
 
     // Check if there already are any content-type meta children.
     // If there are, they will be modified to use the correct charset.
@@ -471,7 +469,7 @@ nsXHTMLContentSerializer::AfterElementStart(nsIContent* aContent,
     for (nsIContent* child = aContent->GetFirstChild();
          child;
          child = child->GetNextSibling()) {
-      if (child->IsHTML(nsGkAtoms::meta) &&
+      if (child->IsHTMLElement(nsGkAtoms::meta) &&
           child->HasAttr(kNameSpaceID_None, nsGkAtoms::content)) {
         nsAutoString header;
         child->GetAttr(kNameSpaceID_None, nsGkAtoms::httpEquiv, header);
@@ -508,12 +506,9 @@ nsXHTMLContentSerializer::AfterElementEnd(nsIContent * aContent,
 {
   NS_ASSERTION(!mIsHTMLSerializer, "nsHTMLContentSerializer shouldn't call this method !");
 
-  int32_t namespaceID = aContent->GetNameSpaceID();
-  nsIAtom *name = aContent->Tag();
-
   // this method is not called by nsHTMLContentSerializer
   // so we don't have to check HTML element, just XHTML
-  if (kNameSpaceID_XHTML == namespaceID && name == nsGkAtoms::body) {
+  if (aContent->IsHTMLElement(nsGkAtoms::body)) {
     --mInBody;
   }
 }
@@ -543,21 +538,17 @@ nsXHTMLContentSerializer::CheckElementStart(nsIContent * aContent,
   aForceFormat = !(mFlags & nsIDocumentEncoder::OutputIgnoreMozDirty) &&
                  aContent->HasAttr(kNameSpaceID_None, nsGkAtoms::mozdirty);
 
-  nsIAtom *name = aContent->Tag();
-  int32_t namespaceID = aContent->GetNameSpaceID();
-
-  if (namespaceID == kNameSpaceID_XHTML) {
-    if (name == nsGkAtoms::br &&
-        (mFlags & nsIDocumentEncoder::OutputNoFormattingInPre) &&
-        PreLevel() > 0) {
-      aResult = AppendNewLineToString(aStr) ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
-      return false;
-    }
-
-    if (name == nsGkAtoms::body) {
-      ++mInBody;
-    }
+  if (aContent->IsHTMLElement(nsGkAtoms::br) &&
+      (mFlags & nsIDocumentEncoder::OutputNoFormattingInPre) &&
+      PreLevel() > 0) {
+    AppendNewLineToString(aStr);
+    return false;
   }
+
+  if (aContent->IsHTMLElement(nsGkAtoms::body)) {
+    ++mInBody;
+  }
+
   return true;
 }
 
@@ -571,13 +562,10 @@ nsXHTMLContentSerializer::CheckElementEnd(nsIContent * aContent,
   aForceFormat = !(mFlags & nsIDocumentEncoder::OutputIgnoreMozDirty) &&
                  aContent->HasAttr(kNameSpaceID_None, nsGkAtoms::mozdirty);
 
-  nsIAtom *name = aContent->Tag();
-  int32_t namespaceID = aContent->GetNameSpaceID();
-
   // this method is not called by nsHTMLContentSerializer
   // so we don't have to check HTML element, just XHTML
-  if (namespaceID == kNameSpaceID_XHTML) {
-    if (mIsCopying && name == nsGkAtoms::ol) {
+  if (aContent->IsHTMLElement()) {
+    if (mIsCopying && aContent->IsHTMLElement(nsGkAtoms::ol)) {
       NS_ASSERTION((!mOLStateStack.IsEmpty()), "Cannot have an empty OL Stack");
       /* Though at this point we must always have an state to be deleted as all 
       the OL opening tags are supposed to push an olState object to the stack*/
@@ -593,7 +581,8 @@ nsXHTMLContentSerializer::CheckElementEnd(nsIContent * aContent,
         bool isContainer;
 
         parserService->
-          IsContainer(parserService->HTMLCaseSensitiveAtomTagToId(name),
+          IsContainer(parserService->HTMLCaseSensitiveAtomTagToId(
+                        aContent->NodeInfo()->NameAtom()),
                       isContainer);
         if (!isContainer) {
           // non-container HTML elements are already closed,
@@ -856,18 +845,15 @@ void
 nsXHTMLContentSerializer::MaybeEnterInPreContent(nsIContent* aNode)
 {
   if (!ShouldMaintainPreLevel() ||
-      aNode->GetNameSpaceID() != kNameSpaceID_XHTML) {
+      !aNode->IsHTMLElement()) {
     return;
   }
 
-  nsIAtom *name = aNode->Tag();
-
   if (IsElementPreformatted(aNode) ||
-      name == nsGkAtoms::script ||
-      name == nsGkAtoms::style ||
-      name == nsGkAtoms::noscript ||
-      name == nsGkAtoms::noframes
-      ) {
+      aNode->IsAnyOfHTMLElements(nsGkAtoms::script,
+                                 nsGkAtoms::style,
+                                 nsGkAtoms::noscript,
+                                 nsGkAtoms::noframes)) {
     PreLevel()++;
   }
 }
@@ -876,17 +862,15 @@ void
 nsXHTMLContentSerializer::MaybeLeaveFromPreContent(nsIContent* aNode)
 {
   if (!ShouldMaintainPreLevel() ||
-      aNode->GetNameSpaceID() != kNameSpaceID_XHTML) {
+      !aNode->IsHTMLElement()) {
     return;
   }
 
-  nsIAtom *name = aNode->Tag();
   if (IsElementPreformatted(aNode) ||
-      name == nsGkAtoms::script ||
-      name == nsGkAtoms::style ||
-      name == nsGkAtoms::noscript ||
-      name == nsGkAtoms::noframes
-    ) {
+      aNode->IsAnyOfHTMLElements(nsGkAtoms::script,
+                                 nsGkAtoms::style,
+                                 nsGkAtoms::noscript,
+                                 nsGkAtoms::noframes)) {
     --PreLevel();
   }
 }
diff --git a/dom/base/nsXMLContentSerializer.cpp b/dom/base/nsXMLContentSerializer.cpp
index 695dbd71df..8c74a70af4 100644
--- a/dom/base/nsXMLContentSerializer.cpp
+++ b/dom/base/nsXMLContentSerializer.cpp
@@ -773,9 +773,9 @@ bool
 nsXMLContentSerializer::IsJavaScript(nsIContent * aContent, nsIAtom* aAttrNameAtom,
                                      int32_t aAttrNamespaceID, const nsAString& aValueString)
 {
-  bool isHtml = aContent->IsHTML();
-  bool isXul = aContent->IsXUL();
-  bool isSvg = aContent->IsSVG();
+  bool isHtml = aContent->IsHTMLElement();
+  bool isXul = aContent->IsXULElement();
+  bool isSvg = aContent->IsSVGElement();
 
   if (aAttrNamespaceID == kNameSpaceID_None &&
       (isHtml || isXul || isSvg) &&
@@ -907,7 +907,7 @@ nsXMLContentSerializer::AppendElementStart(Element* aElement,
   uint32_t skipAttr = ScanNamespaceDeclarations(content,
                           aOriginalElement, tagNamespaceURI);
 
-  nsIAtom *name = content->Tag();
+  nsIAtom *name = content->NodeInfo()->NameAtom();
   bool lineBreakBeforeOpen = LineBreakBeforeOpen(content->GetNameSpaceID(), name);
 
   if ((mDoFormat || forceFormat) && !mDoRaw && !PreLevel()) {
@@ -999,7 +999,7 @@ nsXMLContentSerializer::AppendElementEnd(Element* aElement,
   bool forceFormat = false, outputElementEnd;
   outputElementEnd = CheckElementEnd(content, forceFormat, aStr);
 
-  nsIAtom *name = content->Tag();
+  nsIAtom *name = content->NodeInfo()->NameAtom();
 
   if ((mDoFormat || forceFormat) && !mDoRaw && !PreLevel()) {
     DecrIndentation(name);
diff --git a/dom/events/ContentEventHandler.cpp b/dom/events/ContentEventHandler.cpp
index 22ffd605b0..063c890b98 100644
--- a/dom/events/ContentEventHandler.cpp
+++ b/dom/events/ContentEventHandler.cpp
@@ -195,8 +195,7 @@ ContentEventHandler::QueryContentRect(nsIContent* aContent,
 // we don't want to include the bogus BRs at the end.
 static bool IsContentBR(nsIContent* aContent)
 {
-  return aContent->IsHTML() &&
-         aContent->Tag() == nsGkAtoms::br &&
+  return aContent->IsHTMLElement(nsGkAtoms::br) &&
          !aContent->AttrValueIs(kNameSpaceID_None,
                                 nsGkAtoms::type,
                                 nsGkAtoms::moz,
@@ -1391,8 +1390,8 @@ static void AdjustRangeForSelection(nsIContent* aRoot,
   }
 
   nsIContent* brContent = node->GetChildAt(nodeOffset - 1);
-  while (brContent && brContent->IsHTML()) {
-    if (brContent->Tag() != nsGkAtoms::br || IsContentBR(brContent)) {
+  while (brContent && brContent->IsHTMLElement()) {
+    if (!brContent->IsHTMLElement(nsGkAtoms::br) || IsContentBR(brContent)) {
       break;
     }
     brContent = node->GetChildAt(--nodeOffset - 1);
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index 8473b6e416..23acde9144 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -896,7 +896,7 @@ IsAccessKeyTarget(nsIContent* aContent, nsIFrame* aFrame, nsAString& aKey)
 
   nsCOMPtr<nsIDOMXULDocument> xulDoc =
     do_QueryInterface(aContent->OwnerDoc());
-  if (!xulDoc && !aContent->IsXUL())
+  if (!xulDoc && !aContent->IsXULElement())
     return true;
 
     // For XUL we do visibility checks.
@@ -914,21 +914,18 @@ IsAccessKeyTarget(nsIContent* aContent, nsIFrame* aFrame, nsAString& aKey)
   if (control)
     return true;
 
-  if (aContent->IsHTML()) {
-    nsIAtom* tag = aContent->Tag();
+  // HTML area, label and legend elements are never focusable, so
+  // we need to check for them explicitly before giving up.
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::area,
+                                    nsGkAtoms::label,
+                                    nsGkAtoms::legend)) {
+    return true;
+  }
 
-    // HTML area, label and legend elements are never focusable, so
-    // we need to check for them explicitly before giving up.
-    if (tag == nsGkAtoms::area ||
-        tag == nsGkAtoms::label ||
-        tag == nsGkAtoms::legend)
-      return true;
-
-  } else if (aContent->IsXUL()) {
-    // XUL label elements are never focusable, so we need to check for them
-    // explicitly before giving up.
-    if (aContent->Tag() == nsGkAtoms::label)
-      return true;
+  // XUL label elements are never focusable, so we need to check for them
+  // explicitly before giving up.
+  if (aContent->IsXULElement(nsGkAtoms::label)) {
+    return true;
   }
 
   return false;
@@ -1148,9 +1145,7 @@ EventStateManager::IsRemoteTarget(nsIContent* target) {
   }
 
   // <browser/iframe remote=true> from XUL
-  if ((target->Tag() == nsGkAtoms::browser ||
-       target->Tag() == nsGkAtoms::iframe) &&
-      target->IsXUL() &&
+  if (target->IsAnyOfXULElements(nsGkAtoms::browser, nsGkAtoms::iframe) &&
       target->AttrValueIs(kNameSpaceID_None, nsGkAtoms::Remote,
                           nsGkAtoms::_true, eIgnoreCase)) {
     return true;
@@ -1314,7 +1309,7 @@ EventStateManager::CreateClickHoldTimer(nsPresContext* inPresContext,
       return;
     
     // check for a <menubutton> like bookmarks
-    if (mGestureDownContent->Tag() == nsGkAtoms::menubutton)
+    if (mGestureDownContent->IsXULElement(nsGkAtoms::menubutton))
       return;
   }
 
@@ -1408,40 +1403,37 @@ EventStateManager::FireContextClick()
 
     // before dispatching, check that we're not on something that
     // doesn't get a context menu
-    nsIAtom *tag = mGestureDownContent->Tag();
     bool allowedToDispatch = true;
 
-    if (mGestureDownContent->IsXUL()) {
-      if (tag == nsGkAtoms::scrollbar ||
-          tag == nsGkAtoms::scrollbarbutton ||
-          tag == nsGkAtoms::button)
+    if (mGestureDownContent->IsAnyOfXULElements(nsGkAtoms::scrollbar,
+                                                nsGkAtoms::scrollbarbutton,
+                                                nsGkAtoms::button)) {
+      allowedToDispatch = false;
+    } else if (mGestureDownContent->IsXULElement(nsGkAtoms::toolbarbutton)) {
+      // a <toolbarbutton> that has the container attribute set
+      // will already have its own dropdown.
+      if (nsContentUtils::HasNonEmptyAttr(mGestureDownContent,
+              kNameSpaceID_None, nsGkAtoms::container)) {
         allowedToDispatch = false;
-      else if (tag == nsGkAtoms::toolbarbutton) {
-        // a <toolbarbutton> that has the container attribute set
-        // will already have its own dropdown.
-        if (nsContentUtils::HasNonEmptyAttr(mGestureDownContent,
-                kNameSpaceID_None, nsGkAtoms::container)) {
+      } else {
+        // If the toolbar button has an open menu, don't attempt to open
+          // a second menu
+        if (mGestureDownContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
+                                             nsGkAtoms::_true, eCaseMatters)) {
           allowedToDispatch = false;
-        } else {
-          // If the toolbar button has an open menu, don't attempt to open
-            // a second menu
-          if (mGestureDownContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
-                                               nsGkAtoms::_true, eCaseMatters)) {
-            allowedToDispatch = false;
-          }
         }
       }
     }
-    else if (mGestureDownContent->IsHTML()) {
+    else if (mGestureDownContent->IsHTMLElement()) {
       nsCOMPtr<nsIFormControl> formCtrl(do_QueryInterface(mGestureDownContent));
 
       if (formCtrl) {
         allowedToDispatch = formCtrl->IsTextOrNumberControl(/*aExcludePassword*/ false) ||
                             formCtrl->GetType() == NS_FORM_INPUT_FILE;
       }
-      else if (tag == nsGkAtoms::applet ||
-               tag == nsGkAtoms::embed  ||
-               tag == nsGkAtoms::object) {
+      else if (mGestureDownContent->IsAnyOfHTMLElements(nsGkAtoms::applet,
+                                                        nsGkAtoms::embed,
+                                                        nsGkAtoms::object)) {
         allowedToDispatch = false;
       }
     }
@@ -2680,7 +2672,7 @@ static bool
 NodeAllowsClickThrough(nsINode* aNode)
 {
   while (aNode) {
-    if (aNode->IsElement() && aNode->AsElement()->IsXUL()) {
+    if (aNode->IsXULElement()) {
       mozilla::dom::Element* element = aNode->AsElement();
       static nsIContent::AttrValuesArray strings[] =
         {&nsGkAtoms::always, &nsGkAtoms::never, nullptr};
@@ -2922,7 +2914,7 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
             EnsureDocument(mPresContext);
             if (mDocument) {
 #ifdef XP_MACOSX
-              if (!activeContent || !activeContent->IsXUL())
+              if (!activeContent || !activeContent->IsXULElement())
 #endif
                 fm->ClearFocus(mDocument->GetWindow());
               fm->SetFocusedWindow(mDocument->GetWindow());
@@ -4876,8 +4868,7 @@ EventStateManager::ContentRemoved(nsIDocument* aDocument, nsIContent* aContent)
    * the current link. We want to make sure that the UI gets informed when they
    * are actually removed from the DOM.
    */
-  if (aContent->IsHTML() &&
-      (aContent->Tag() == nsGkAtoms::a || aContent->Tag() == nsGkAtoms::area) &&
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::a, nsGkAtoms::area) &&
       (aContent->AsElement()->State().HasAtLeastOneOfStates(NS_EVENT_STATE_FOCUS |
                                                             NS_EVENT_STATE_HOVER))) {
     nsGenericHTMLElement* element = static_cast<nsGenericHTMLElement*>(aContent);
diff --git a/dom/events/IMEContentObserver.cpp b/dom/events/IMEContentObserver.cpp
index 5535a04334..ba6f7f769b 100644
--- a/dom/events/IMEContentObserver.cpp
+++ b/dom/events/IMEContentObserver.cpp
@@ -917,7 +917,7 @@ GetContentBR(dom::Element* aElement)
     return nullptr;
   }
   nsIContent* content = static_cast<nsIContent*>(aElement);
-  return content->IsHTML(nsGkAtoms::br) ? content : nullptr;
+  return content->IsHTMLElement(nsGkAtoms::br) ? content : nullptr;
 }
 
 void
diff --git a/dom/events/IMEStateManager.cpp b/dom/events/IMEStateManager.cpp
index 6c9d957c85..a4d9146bc3 100644
--- a/dom/events/IMEStateManager.cpp
+++ b/dom/events/IMEStateManager.cpp
@@ -791,10 +791,9 @@ IMEStateManager::SetIMEState(const IMEState& aState,
   InputContext context;
   context.mIMEState = aState;
 
-  if (aContent && aContent->GetNameSpaceID() == kNameSpaceID_XHTML &&
-      (aContent->Tag() == nsGkAtoms::input ||
-       aContent->Tag() == nsGkAtoms::textarea)) {
-    if (aContent->Tag() != nsGkAtoms::textarea) {
+  if (aContent &&
+      aContent->IsAnyOfHTMLElements(nsGkAtoms::input, nsGkAtoms::textarea)) {
+    if (!aContent->IsHTMLElement(nsGkAtoms::textarea)) {
       // <input type=number> has an anonymous <input type=text> descendant
       // that gets focus whenever anyone tries to focus the number control. We
       // need to check if aContent is one of those anonymous text controls and,
@@ -830,7 +829,7 @@ IMEStateManager::SetIMEState(const IMEState& aState,
     // If we don't have an action hint and
     // return won't submit the form, use "next".
     if (context.mActionHint.IsEmpty() &&
-        inputContent->Tag() == nsGkAtoms::input) {
+        inputContent->IsHTMLElement(nsGkAtoms::input)) {
       bool willSubmit = false;
       nsCOMPtr<nsIFormControl> control(do_QueryInterface(inputContent));
       mozilla::dom::Element* formElement = control->GetFormElement();
@@ -841,8 +840,7 @@ IMEStateManager::SetIMEState(const IMEState& aState,
             form->GetDefaultSubmitElement()) {
           willSubmit = true;
         // is this an html form and does it only have a single text input element?
-        } else if (formElement && formElement->Tag() == nsGkAtoms::form &&
-                   formElement->IsHTML() &&
+        } else if (formElement && formElement->IsHTMLElement(nsGkAtoms::form) &&
                    !static_cast<dom::HTMLFormElement*>(formElement)->
                      ImplicitSubmissionIsDisabled()) {
           willSubmit = true;
diff --git a/dom/html/HTMLAllCollection.cpp b/dom/html/HTMLAllCollection.cpp
index 2a0bd18802..80acfeae0b 100644
--- a/dom/html/HTMLAllCollection.cpp
+++ b/dom/html/HTMLAllCollection.cpp
@@ -68,23 +68,21 @@ HTMLAllCollection::Collection()
 static bool
 IsAllNamedElement(nsIContent* aContent)
 {
-  nsIAtom* tag = aContent->Tag();
-  return
-    tag == nsGkAtoms::a ||
-    tag == nsGkAtoms::applet ||
-    tag == nsGkAtoms::button ||
-    tag == nsGkAtoms::embed ||
-    tag == nsGkAtoms::form ||
-    tag == nsGkAtoms::iframe ||
-    tag == nsGkAtoms::img ||
-    tag == nsGkAtoms::input ||
-    tag == nsGkAtoms::map ||
-    tag == nsGkAtoms::meta ||
-    tag == nsGkAtoms::object ||
-    tag == nsGkAtoms::select ||
-    tag == nsGkAtoms::textarea ||
-    tag == nsGkAtoms::frame ||
-    tag == nsGkAtoms::frameset;
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::a,
+                                       nsGkAtoms::applet,
+                                       nsGkAtoms::button,
+                                       nsGkAtoms::embed,
+                                       nsGkAtoms::form,
+                                       nsGkAtoms::iframe,
+                                       nsGkAtoms::img,
+                                       nsGkAtoms::input,
+                                       nsGkAtoms::map,
+                                       nsGkAtoms::meta,
+                                       nsGkAtoms::object,
+                                       nsGkAtoms::select,
+                                       nsGkAtoms::textarea,
+                                       nsGkAtoms::frame,
+                                       nsGkAtoms::frameset);
 }
 
 static bool
diff --git a/dom/html/HTMLContentElement.cpp b/dom/html/HTMLContentElement.cpp
index d9f690b6c0..2a134c747d 100644
--- a/dom/html/HTMLContentElement.cpp
+++ b/dom/html/HTMLContentElement.cpp
@@ -65,8 +65,7 @@ HTMLContentElement::BindToTree(nsIDocument* aDocument,
   if (containingShadow && !oldContainingShadow) {
     nsINode* parentNode = nsINode::GetParentNode();
     while (parentNode && parentNode != containingShadow) {
-      if (parentNode->IsElement() &&
-          parentNode->AsElement()->IsHTML(nsGkAtoms::content)) {
+      if (parentNode->IsHTMLElement(nsGkAtoms::content)) {
         // Content element in fallback content is not an insertion point.
         return NS_OK;
       }
diff --git a/dom/html/HTMLDetailsElement.cpp b/dom/html/HTMLDetailsElement.cpp
index ff1bac93b2..ef0cd6800a 100644
--- a/dom/html/HTMLDetailsElement.cpp
+++ b/dom/html/HTMLDetailsElement.cpp
@@ -29,7 +29,7 @@ HTMLDetailsElement::GetFirstSummary() const
   for (nsIContent* child = nsINode::GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(nsGkAtoms::summary)) {
+    if (child->IsHTMLElement(nsGkAtoms::summary)) {
       return child;
     }
   }
diff --git a/dom/html/HTMLFieldSetElement.cpp b/dom/html/HTMLFieldSetElement.cpp
index 081dbdd69c..25464e4335 100644
--- a/dom/html/HTMLFieldSetElement.cpp
+++ b/dom/html/HTMLFieldSetElement.cpp
@@ -168,7 +168,7 @@ HTMLFieldSetElement::InsertChildAt(nsIContent* aChild, uint32_t aIndex,
 {
   bool firstLegendHasChanged = false;
 
-  if (aChild->IsHTML(nsGkAtoms::legend)) {
+  if (aChild->IsHTMLElement(nsGkAtoms::legend)) {
     if (!mFirstLegend) {
       mFirstLegend = aChild;
       // We do not want to notify the first time mFirstElement is set.
@@ -204,7 +204,7 @@ HTMLFieldSetElement::RemoveChildAt(uint32_t aIndex, bool aNotify)
     firstLegendHasChanged = true;
 
     for (; child; child = child->GetNextSibling()) {
-      if (child->IsHTML(nsGkAtoms::legend)) {
+      if (child->IsHTMLElement(nsGkAtoms::legend)) {
         mFirstLegend = child;
         break;
       }
diff --git a/dom/html/HTMLFormElement.cpp b/dom/html/HTMLFormElement.cpp
index 50f04b706c..c08531cc0b 100644
--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -704,7 +704,7 @@ HTMLFormElement::BuildSubmission(nsFormSubmission** aFormSubmission,
     if (formEvent) {
       nsIContent* originator = formEvent->originator;
       if (originator) {
-        if (!originator->IsHTML()) {
+        if (!originator->IsHTMLElement()) {
           return NS_ERROR_UNEXPECTED;
         }
         originatingElement = static_cast<nsGenericHTMLElement*>(originator);
@@ -1968,7 +1968,7 @@ HTMLFormElement::CheckValidFormSubmission()
              i < length; ++i) {
           // Input elements can trigger a form submission and we want to
           // update the style in that case.
-          if (mControls->mElements[i]->IsHTML(nsGkAtoms::input) &&
+          if (mControls->mElements[i]->IsHTMLElement(nsGkAtoms::input) &&
               nsContentUtils::IsFocusedContent(mControls->mElements[i])) {
             static_cast<HTMLInputElement*>(mControls->mElements[i])
               ->UpdateValidityUIBits(true);
diff --git a/dom/html/HTMLImageElement.cpp b/dom/html/HTMLImageElement.cpp
index a3178a6cc7..80450b0290 100644
--- a/dom/html/HTMLImageElement.cpp
+++ b/dom/html/HTMLImageElement.cpp
@@ -586,7 +586,7 @@ HTMLImageElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
     UpdateFormOwner();
   }
 
-  bool addedToPicture = aParent && aParent->IsHTML(nsGkAtoms::picture) &&
+  bool addedToPicture = aParent && aParent->IsHTMLElement(nsGkAtoms::picture) &&
                         HTMLPictureElement::IsPictureEnabled();
   if (addedToPicture) {
     QueueImageLoadTask();
@@ -631,7 +631,7 @@ HTMLImageElement::UnbindFromTree(bool aDeep, bool aNullParent)
   }
 
   if (aNullParent && GetParent() &&
-      GetParent()->IsHTML(nsGkAtoms::picture) &&
+      GetParent()->IsHTMLElement(nsGkAtoms::picture) &&
       HTMLPictureElement::IsPictureEnabled()) {
     // Being removed from picture re-triggers selection, even if we
     // weren't using a <source> peer
@@ -889,7 +889,7 @@ HTMLImageElement::HaveSrcsetOrInPicture()
   }
 
   Element *parent = nsINode::GetParentElement();
-  return (parent && parent->IsHTML(nsGkAtoms::picture));
+  return (parent && parent->IsHTMLElement(nsGkAtoms::picture));
 }
 
 bool
@@ -1047,7 +1047,7 @@ HTMLImageElement::UpdateResponsiveSource()
   Element *parent = pictureEnabled ? nsINode::GetParentElement() : nullptr;
 
   nsINode *candidateSource = nullptr;
-  if (parent && parent->IsHTML(nsGkAtoms::picture)) {
+  if (parent && parent->IsHTMLElement(nsGkAtoms::picture)) {
     // Walk source nodes previous to ourselves
     candidateSource = parent->GetFirstChild();
   } else {
@@ -1076,8 +1076,7 @@ HTMLImageElement::UpdateResponsiveSource()
         mResponsiveSelector = nullptr;
       }
       break;
-    } else if (candidateSource->IsElement() &&
-               candidateSource->AsElement()->IsHTML(nsGkAtoms::source) &&
+    } else if (candidateSource->IsHTMLElement(nsGkAtoms::source) &&
                TryCreateResponsiveSelector(candidateSource->AsContent())) {
       // This led to a valid source, stop
       break;
@@ -1110,10 +1109,10 @@ HTMLImageElement::TryCreateResponsiveSelector(nsIContent *aSourceNode,
 
   bool pictureEnabled = HTMLPictureElement::IsPictureEnabled();
   // Skip if this is not a <source> with matching media query
-  bool isSourceTag = aSourceNode->IsHTML(nsGkAtoms::source);
+  bool isSourceTag = aSourceNode->IsHTMLElement(nsGkAtoms::source);
   if (isSourceTag) {
     DebugOnly<Element *> parent(nsINode::GetParentElement());
-    MOZ_ASSERT(parent && parent->IsHTML(nsGkAtoms::picture));
+    MOZ_ASSERT(parent && parent->IsHTMLElement(nsGkAtoms::picture));
     MOZ_ASSERT(IsPreviousSibling(aSourceNode, this));
     MOZ_ASSERT(pictureEnabled);
 
@@ -1128,7 +1127,7 @@ HTMLImageElement::TryCreateResponsiveSelector(nsIContent *aSourceNode,
         !SupportedPictureSourceType(type)) {
       return false;
     }
-  } else if (aSourceNode->IsHTML(nsGkAtoms::img)) {
+  } else if (aSourceNode->IsHTMLElement(nsGkAtoms::img)) {
     // Otherwise this is the <img> tag itself
     MOZ_ASSERT(aSourceNode == this);
   }
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 8c1837fc5d..51bbc37208 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -1898,7 +1898,7 @@ HTMLInputElement::GetList() const
   }
 
   Element* element = doc->GetElementById(dataListId);
-  if (!element || !element->IsHTML(nsGkAtoms::datalist)) {
+  if (!element || !element->IsHTMLElement(nsGkAtoms::datalist)) {
     return nullptr;
   }
 
diff --git a/dom/html/HTMLLegendElement.cpp b/dom/html/HTMLLegendElement.cpp
index 69b0ee49ea..cd2fa9930b 100644
--- a/dom/html/HTMLLegendElement.cpp
+++ b/dom/html/HTMLLegendElement.cpp
@@ -26,7 +26,7 @@ HTMLLegendElement::GetFieldSet() const
 {
   nsIContent* parent = GetParent();
 
-  if (parent && parent->IsHTML(nsGkAtoms::fieldset)) {
+  if (parent && parent->IsHTMLElement(nsGkAtoms::fieldset)) {
     return parent;
   }
 
@@ -140,7 +140,7 @@ already_AddRefed<HTMLFormElement>
 HTMLLegendElement::GetForm()
 {
   Element* form = GetFormElement();
-  MOZ_ASSERT_IF(form, form->IsHTML(nsGkAtoms::form));
+  MOZ_ASSERT_IF(form, form->IsHTMLElement(nsGkAtoms::form));
   nsRefPtr<HTMLFormElement> ret = static_cast<HTMLFormElement*>(form);
   return ret.forget();
 }
diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index a6a6c1b64d..c0997d913f 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -818,7 +818,7 @@ static bool HasSourceChildren(nsIContent* aElement)
   for (nsIContent* child = aElement->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(nsGkAtoms::source))
+    if (child->IsHTMLElement(nsGkAtoms::source))
     {
       return true;
     }
@@ -4150,7 +4150,7 @@ nsIContent* HTMLMediaElement::GetNextSource()
     nsIContent* child = GetChildAt(startOffset);
 
     // If child is a <source> element, it is the next candidate.
-    if (child && child->IsHTML(nsGkAtoms::source)) {
+    if (child && child->IsHTMLElement(nsGkAtoms::source)) {
       mSourceLoadCandidate = child;
       return child;
     }
diff --git a/dom/html/HTMLMenuElement.cpp b/dom/html/HTMLMenuElement.cpp
index 267fcabb14..a778d8cea0 100644
--- a/dom/html/HTMLMenuElement.cpp
+++ b/dom/html/HTMLMenuElement.cpp
@@ -206,9 +206,7 @@ HTMLMenuElement::TraverseContent(nsIContent* aContent,
       continue;
     }
 
-    nsIAtom* tag = child->Tag();
-
-    if (tag == nsGkAtoms::menuitem) {
+    if (child->IsHTMLElement(nsGkAtoms::menuitem)) {
       HTMLMenuItemElement* menuitem = HTMLMenuItemElement::FromContent(child);
 
       if (menuitem->IsHidden()) {
@@ -227,9 +225,7 @@ HTMLMenuElement::TraverseContent(nsIContent* aContent,
       aBuilder->AddItemFor(menuitem, CanLoadIcon(child, icon));
 
       aSeparator = ST_FALSE;
-    } else if (child->IsHTML(nsGkAtoms::hr)) {
-      aBuilder->AddSeparator();
-    } else if (tag == nsGkAtoms::menu && !element->IsHidden()) {
+    } else if (child->IsHTMLElement(nsGkAtoms::menu) && !element->IsHidden()) {
       if (child->HasAttr(kNameSpaceID_None, nsGkAtoms::label)) {
         nsAutoString label;
         child->GetAttr(kNameSpaceID_None, nsGkAtoms::label, label);
diff --git a/dom/html/HTMLOptGroupElement.cpp b/dom/html/HTMLOptGroupElement.cpp
index 72d71bc7bc..1532e647fa 100644
--- a/dom/html/HTMLOptGroupElement.cpp
+++ b/dom/html/HTMLOptGroupElement.cpp
@@ -72,11 +72,11 @@ nsIContent*
 HTMLOptGroupElement::GetSelect()
 {
   nsIContent* parent = this;
-  while ((parent = parent->GetParent()) && parent->IsHTML()) {
-    if (parent->Tag() == nsGkAtoms::select) {
+  while ((parent = parent->GetParent()) && parent->IsHTMLElement()) {
+    if (parent->IsHTMLElement(nsGkAtoms::select)) {
       return parent;
     }
-    if (parent->Tag() != nsGkAtoms::optgroup) {
+    if (!parent->IsHTMLElement(nsGkAtoms::optgroup)) {
       break;
     }
   }
diff --git a/dom/html/HTMLOptionElement.cpp b/dom/html/HTMLOptionElement.cpp
index 0db074ac43..17f2dcdac9 100644
--- a/dom/html/HTMLOptionElement.cpp
+++ b/dom/html/HTMLOptionElement.cpp
@@ -311,7 +311,8 @@ HTMLOptionElement::GetText(nsAString& aText)
         child->NodeType() == nsIDOMNode::CDATA_SECTION_NODE) {
       child->AppendTextTo(text);
     }
-    if (child->IsHTML(nsGkAtoms::script) || child->IsSVG(nsGkAtoms::script)) {
+    if (child->IsHTMLElement(nsGkAtoms::script) ||
+        child->IsSVGElement(nsGkAtoms::script)) {
       child = child->GetNextNonChildNode(this);
     } else {
       child = child->GetNextNode(this);
@@ -376,12 +377,12 @@ HTMLOptionElement::GetSelect()
 {
   nsIContent* parent = this;
   while ((parent = parent->GetParent()) &&
-         parent->IsHTML()) {
+         parent->IsHTMLElement()) {
     HTMLSelectElement* select = HTMLSelectElement::FromContent(parent);
     if (select) {
       return select;
     }
-    if (parent->Tag() != nsGkAtoms::optgroup) {
+    if (!parent->IsHTMLElement(nsGkAtoms::optgroup)) {
       break;
     }
   }
diff --git a/dom/html/HTMLPictureElement.cpp b/dom/html/HTMLPictureElement.cpp
index b8a4bae342..0d4d8e5d84 100644
--- a/dom/html/HTMLPictureElement.cpp
+++ b/dom/html/HTMLPictureElement.cpp
@@ -46,7 +46,7 @@ HTMLPictureElement::RemoveChildAt(uint32_t aIndex, bool aNotify)
   // Find all img siblings after this <source> to notify them of its demise
   nsCOMPtr<nsIContent> child = GetChildAt(aIndex);
   nsCOMPtr<nsIContent> nextSibling;
-  if (child && child->IsHTML(nsGkAtoms::source)) {
+  if (child && child->IsHTMLElement(nsGkAtoms::source)) {
     nextSibling = child->GetNextSibling();
   }
 
diff --git a/dom/html/HTMLPropertiesCollection.cpp b/dom/html/HTMLPropertiesCollection.cpp
index 1a011c3b74..664863433b 100644
--- a/dom/html/HTMLPropertiesCollection.cpp
+++ b/dom/html/HTMLPropertiesCollection.cpp
@@ -277,11 +277,11 @@ HTMLPropertiesCollection::CrawlSubtree(Element* aElement)
   while (aContent) {
     // We must check aContent against mRoot because 
     // an element must not be its own property
-    if (aContent == mRoot || !aContent->IsHTML()) {
+    if (aContent == mRoot || !aContent->IsHTMLElement()) {
       // Move on to the next node in the tree
       aContent = aContent->GetNextNode(aElement);
     } else {
-      MOZ_ASSERT(aContent->IsElement(), "IsHTML() returned true!");
+      MOZ_ASSERT(aContent->IsElement(), "IsHTMLElement() returned true!");
       Element* element = aContent->AsElement();
       if (element->HasAttr(kNameSpaceID_None, nsGkAtoms::itemprop) &&
           !mProperties.Contains(element)) {
diff --git a/dom/html/HTMLSelectElement.cpp b/dom/html/HTMLSelectElement.cpp
index 21ca9e1ef2..428605869f 100644
--- a/dom/html/HTMLSelectElement.cpp
+++ b/dom/html/HTMLSelectElement.cpp
@@ -350,7 +350,7 @@ HTMLSelectElement::InsertOptionsIntoListRecurse(nsIContent* aOptions,
   }
 
   // Recurse down into optgroups
-  if (aOptions->IsHTML(nsGkAtoms::optgroup)) {
+  if (aOptions->IsHTMLElement(nsGkAtoms::optgroup)) {
     mOptGroupCount++;
 
     for (nsIContent* child = aOptions->GetFirstChild();
@@ -391,7 +391,7 @@ HTMLSelectElement::RemoveOptionsFromListRecurse(nsIContent* aOptions,
   }
 
   // Recurse down deeper for options
-  if (mOptGroupCount && aOptions->IsHTML(nsGkAtoms::optgroup)) {
+  if (mOptGroupCount && aOptions->IsHTMLElement(nsGkAtoms::optgroup)) {
     mOptGroupCount--;
 
     for (nsIContent* child = aOptions->GetFirstChild();
@@ -1136,7 +1136,7 @@ HTMLSelectElement::IsOptionDisabled(HTMLOptionElement* aOption)
          node;
          node = node->GetParentElement()) {
       // If we reached the select element, we're done
-      if (node->IsHTML(nsGkAtoms::select)) {
+      if (node->IsHTMLElement(nsGkAtoms::select)) {
         return false;
       }
 
@@ -1801,7 +1801,7 @@ AddOptionsRecurse(nsIContent* aRoot, HTMLOptionsCollection* aArray)
     HTMLOptionElement* opt = HTMLOptionElement::FromContent(cur);
     if (opt) {
       aArray->AppendOption(opt);
-    } else if (cur->IsHTML(nsGkAtoms::optgroup)) {
+    } else if (cur->IsHTMLElement(nsGkAtoms::optgroup)) {
       AddOptionsRecurse(cur, aArray);
     }
   }
@@ -1882,7 +1882,7 @@ VerifyOptionsRecurse(nsIContent* aRoot, int32_t& aIndex,
     if (opt) {
       NS_ASSERTION(opt == aArray->ItemAsOption(aIndex++),
                    "Options collection broken");
-    } else if (cur->IsHTML(nsGkAtoms::optgroup)) {
+    } else if (cur->IsHTMLElement(nsGkAtoms::optgroup)) {
       VerifyOptionsRecurse(cur, aIndex, aArray);
     }
   }
diff --git a/dom/html/HTMLShadowElement.cpp b/dom/html/HTMLShadowElement.cpp
index 4ae42212c2..8afda014f5 100644
--- a/dom/html/HTMLShadowElement.cpp
+++ b/dom/html/HTMLShadowElement.cpp
@@ -94,8 +94,7 @@ IsInFallbackContent(nsIContent* aContent)
 {
   nsINode* parentNode = aContent->GetParentNode();
   while (parentNode) {
-    if (parentNode->IsElement() &&
-        parentNode->AsElement()->IsHTML(nsGkAtoms::content)) {
+    if (parentNode->IsHTMLElement(nsGkAtoms::content)) {
       return true;
     }
     parentNode = parentNode->GetParentNode();
diff --git a/dom/html/HTMLSharedElement.cpp b/dom/html/HTMLSharedElement.cpp
index 3ba78e8ae2..cf551333da 100644
--- a/dom/html/HTMLSharedElement.cpp
+++ b/dom/html/HTMLSharedElement.cpp
@@ -157,7 +157,7 @@ SetBaseURIUsingFirstBaseWithHref(nsIDocument* aDocument, nsIContent* aMustMatch)
 
   for (nsIContent* child = aDocument->GetFirstChild(); child;
        child = child->GetNextNode()) {
-    if (child->IsHTML(nsGkAtoms::base) &&
+    if (child->IsHTMLElement(nsGkAtoms::base) &&
         child->HasAttr(kNameSpaceID_None, nsGkAtoms::href)) {
       if (aMustMatch && child != aMustMatch) {
         return;
@@ -194,7 +194,7 @@ SetBaseTargetUsingFirstBaseWithTarget(nsIDocument* aDocument,
 
   for (nsIContent* child = aDocument->GetFirstChild(); child;
        child = child->GetNextNode()) {
-    if (child->IsHTML(nsGkAtoms::base) &&
+    if (child->IsHTMLElement(nsGkAtoms::base) &&
         child->HasAttr(kNameSpaceID_None, nsGkAtoms::target)) {
       if (aMustMatch && child != aMustMatch) {
         return;
diff --git a/dom/html/HTMLSourceElement.cpp b/dom/html/HTMLSourceElement.cpp
index 9d7f414d60..17dfd14106 100644
--- a/dom/html/HTMLSourceElement.cpp
+++ b/dom/html/HTMLSourceElement.cpp
@@ -86,12 +86,12 @@ HTMLSourceElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
        aName == nsGkAtoms::sizes ||
        aName == nsGkAtoms::media ||
        aName == nsGkAtoms::type) &&
-      parent && parent->IsHTML(nsGkAtoms::picture)) {
+      parent && parent->IsHTMLElement(nsGkAtoms::picture)) {
     nsString strVal = aValue ? aValue->GetStringValue() : EmptyString();
     // Find all img siblings after this <source> and notify them of the change
     nsCOMPtr<nsIContent> sibling = AsContent();
     while ( (sibling = sibling->GetNextSibling()) ) {
-      if (sibling->IsHTML(nsGkAtoms::img)) {
+      if (sibling->IsHTMLElement(nsGkAtoms::img)) {
         HTMLImageElement *img = static_cast<HTMLImageElement*>(sibling.get());
         if (aName == nsGkAtoms::srcset) {
           img->PictureSourceSrcsetChanged(AsContent(), strVal, aNotify);
@@ -147,11 +147,11 @@ HTMLSourceElement::BindToTree(nsIDocument *aDocument,
   if (aParent && aParent->IsNodeOfType(nsINode::eMEDIA)) {
     HTMLMediaElement* media = static_cast<HTMLMediaElement*>(aParent);
     media->NotifyAddedSource();
-  } else if (aParent && aParent->IsHTML(nsGkAtoms::picture)) {
+  } else if (aParent && aParent->IsHTMLElement(nsGkAtoms::picture)) {
     // Find any img siblings after this <source> and notify them
     nsCOMPtr<nsIContent> sibling = AsContent();
     while ( (sibling = sibling->GetNextSibling()) ) {
-      if (sibling->IsHTML(nsGkAtoms::img)) {
+      if (sibling->IsHTMLElement(nsGkAtoms::img)) {
         HTMLImageElement *img = static_cast<HTMLImageElement*>(sibling.get());
         img->PictureSourceAdded(AsContent());
       }
diff --git a/dom/html/HTMLTableCellElement.cpp b/dom/html/HTMLTableCellElement.cpp
index 1f18bac477..ec9e0bf494 100644
--- a/dom/html/HTMLTableCellElement.cpp
+++ b/dom/html/HTMLTableCellElement.cpp
@@ -56,14 +56,14 @@ HTMLTableCellElement::GetTable() const
     return nullptr;
   }
 
-  if (section->IsHTML(nsGkAtoms::table)) {
+  if (section->IsHTMLElement(nsGkAtoms::table)) {
     // XHTML, without a row group.
     return static_cast<HTMLTableElement*>(section);
   }
 
   // We have a row group.
   nsIContent* result = section->GetParent();
-  if (result && result->IsHTML(nsGkAtoms::table)) {
+  if (result && result->IsHTMLElement(nsGkAtoms::table)) {
     return static_cast<HTMLTableElement*>(result);
   }
 
diff --git a/dom/html/HTMLTableElement.cpp b/dom/html/HTMLTableElement.cpp
index 29a47d1555..be8193fd1f 100644
--- a/dom/html/HTMLTableElement.cpp
+++ b/dom/html/HTMLTableElement.cpp
@@ -119,7 +119,7 @@ NS_INTERFACE_MAP_END
       /* TBodies */                                                  \
       for (nsIContent* _node = mParent->nsINode::GetFirstChild();    \
            _node; _node = _node->GetNextSibling()) {                 \
-        if (_node->IsHTML(nsGkAtoms::tbody)) {                       \
+        if (_node->IsHTMLElement(nsGkAtoms::tbody)) {                \
           rowGroup = static_cast<HTMLTableSectionElement*>(_node);   \
           rows = rowGroup->Rows();                                   \
           do { /* gives scoping */                                   \
@@ -475,7 +475,7 @@ HTMLTableElement::CreateTBody()
   for (nsIContent* child = nsINode::GetLastChild();
        child;
        child = child->GetPreviousSibling()) {
-    if (child->IsHTML(nsGkAtoms::tbody)) {
+    if (child->IsHTMLElement(nsGkAtoms::tbody)) {
       referenceNode = child->GetNextSibling();
       break;
     }
@@ -553,7 +553,7 @@ HTMLTableElement::InsertRow(int32_t aIndex, ErrorResult& aError)
     for (nsIContent* child = nsINode::GetLastChild();
          child;
          child = child->GetPreviousSibling()) {
-      if (child->IsHTML(nsGkAtoms::tbody)) {
+      if (child->IsHTMLElement(nsGkAtoms::tbody)) {
         rowGroup = child;
         break;
       }
diff --git a/dom/html/HTMLTableElement.h b/dom/html/HTMLTableElement.h
index 4a1370a9c8..db93a2b636 100644
--- a/dom/html/HTMLTableElement.h
+++ b/dom/html/HTMLTableElement.h
@@ -54,7 +54,7 @@ public:
   }
   void SetTHead(HTMLTableSectionElement* aTHead, ErrorResult& aError)
   {
-    if (aTHead && !aTHead->IsHTML(nsGkAtoms::thead)) {
+    if (aTHead && !aTHead->IsHTMLElement(nsGkAtoms::thead)) {
       aError.Throw(NS_ERROR_DOM_HIERARCHY_REQUEST_ERR);
       return;
     }
@@ -75,7 +75,7 @@ public:
   }
   void SetTFoot(HTMLTableSectionElement* aTFoot, ErrorResult& aError)
   {
-    if (aTFoot && !aTFoot->IsHTML(nsGkAtoms::tfoot)) {
+    if (aTFoot && !aTFoot->IsHTMLElement(nsGkAtoms::tfoot)) {
       aError.Throw(NS_ERROR_DOM_HIERARCHY_REQUEST_ERR);
       return;
     }
@@ -213,7 +213,7 @@ protected:
   {
     for (nsIContent* cur = nsINode::GetFirstChild(); cur;
          cur = cur->GetNextSibling()) {
-      if (cur->IsHTML(aTag)) {
+      if (cur->IsHTMLElement(aTag)) {
         return cur;
       }
     }
diff --git a/dom/html/HTMLTableRowElement.cpp b/dom/html/HTMLTableRowElement.cpp
index 0ffc0ac683..5f2c2a2753 100644
--- a/dom/html/HTMLTableRowElement.cpp
+++ b/dom/html/HTMLTableRowElement.cpp
@@ -51,9 +51,10 @@ HTMLTableSectionElement*
 HTMLTableRowElement::GetSection() const
 {
   nsIContent* parent = GetParent();
-  if (parent && parent->IsHTML() && (parent->Tag() == nsGkAtoms::thead ||
-                                     parent->Tag() == nsGkAtoms::tbody ||
-                                     parent->Tag() == nsGkAtoms::tfoot)) {
+  if (parent &&
+      parent->IsAnyOfHTMLElements(nsGkAtoms::thead,
+                                  nsGkAtoms::tbody,
+                                  nsGkAtoms::tfoot)) {
     return static_cast<HTMLTableSectionElement*>(parent);
   }
   return nullptr;
@@ -121,10 +122,7 @@ static bool
 IsCell(nsIContent *aContent, int32_t aNamespaceID,
        nsIAtom* aAtom, void *aData)
 {
-  nsIAtom* tag = aContent->Tag();
-
-  return ((tag == nsGkAtoms::td || tag == nsGkAtoms::th) &&
-          aContent->IsHTML());
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th);
 }
 
 nsIHTMLCollection*
diff --git a/dom/html/ImageDocument.cpp b/dom/html/ImageDocument.cpp
index e3dd187e25..b9f9ee4ffd 100644
--- a/dom/html/ImageDocument.cpp
+++ b/dom/html/ImageDocument.cpp
@@ -641,7 +641,7 @@ ImageDocument::CreateSyntheticDocument()
     return NS_ERROR_FAILURE;
   }
 
-  nsRefPtr<NodeInfo> nodeInfo;
+  nsRefPtr<mozilla::dom::NodeInfo> nodeInfo;
   nodeInfo = mNodeInfoManager->GetNodeInfo(nsGkAtoms::img, nullptr,
                                            kNameSpaceID_XHTML,
                                            nsIDOMNode::ELEMENT_NODE);
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index 51cb58f231..3171cec634 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -317,8 +317,9 @@ IsOffsetParent(nsIFrame* aFrame)
     // with display: table-cell with no actual table
     nsIContent* content = aFrame->GetContent();
 
-    return content->IsHTML(nsGkAtoms::table) || content->IsHTML(nsGkAtoms::td)
-      || content->IsHTML(nsGkAtoms::th);
+    return content->IsAnyOfHTMLElements(nsGkAtoms::table,
+                                        nsGkAtoms::td,
+                                        nsGkAtoms::th);
   }
   return false;
 }
@@ -346,7 +347,8 @@ nsGenericHTMLElement::GetOffsetRect(CSSIntRect& aRect)
   Element* docElement = GetComposedDoc()->GetRootElement();
   nsIContent* content = frame->GetContent();
 
-  if (content && (content->IsHTML(nsGkAtoms::body) || content == docElement)) {
+  if (content && (content->IsHTMLElement(nsGkAtoms::body) ||
+                  content == docElement)) {
     parent = frame;
   }
   else {
@@ -378,7 +380,7 @@ nsGenericHTMLElement::GetOffsetRect(CSSIntRect& aRect)
 
         // Break if the ancestor frame type makes it suitable as offset parent
         // and this element is *not* positioned or if we found the body element.
-        if (isOffsetParent || content->IsHTML(nsGkAtoms::body)) {
+        if (isOffsetParent || content->IsHTMLElement(nsGkAtoms::body)) {
           offsetParent = content;
           break;
         }
@@ -438,7 +440,7 @@ nsGenericHTMLElement::Spellcheck()
   // Has the state has been explicitly set?
   nsIContent* node;
   for (node = this; node; node = node->GetParent()) {
-    if (node->IsHTML()) {
+    if (node->IsHTMLElement()) {
       static nsIContent::AttrValuesArray strings[] =
         {&nsGkAtoms::_true, &nsGkAtoms::_false, nullptr};
       switch (node->FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::spellcheck,
@@ -599,7 +601,7 @@ HTMLFormElement*
 nsGenericHTMLElement::FindAncestorForm(HTMLFormElement* aCurrentForm)
 {
   NS_ASSERTION(!HasAttr(kNameSpaceID_None, nsGkAtoms::form) ||
-               IsHTML(nsGkAtoms::img),
+               IsHTMLElement(nsGkAtoms::img),
                "FindAncestorForm should not be called if @form is set!");
 
   // Make sure we don't end up finding a form that's anonymous from
@@ -609,7 +611,7 @@ nsGenericHTMLElement::FindAncestorForm(HTMLFormElement* aCurrentForm)
   nsIContent* content = this;
   while (content != bindingParent && content) {
     // If the current ancestor is a form, return it as our form
-    if (content->IsHTML(nsGkAtoms::form)) {
+    if (content->IsHTMLElement(nsGkAtoms::form)) {
 #ifdef DEBUG
       if (!nsContentUtils::IsInSameAnonymousTree(this, content)) {
         // It's possible that we started unbinding at |content| or
@@ -665,7 +667,8 @@ nsGenericHTMLElement::CheckHandleEventForAnchorsPreconditions(
   nsCOMPtr<nsIContent> target = aVisitor.mPresContext->EventStateManager()->
     GetEventTargetContent(aVisitor.mEvent);
 
-  return !target || !target->IsHTML(nsGkAtoms::area) || IsHTML(nsGkAtoms::area);
+  return !target || !target->IsHTMLElement(nsGkAtoms::area) ||
+         IsHTMLElement(nsGkAtoms::area);
 }
 
 nsresult
@@ -809,7 +812,7 @@ nsGenericHTMLElement::GetEventListenerManagerForAttr(nsIAtom* aAttrName,
 EventHandlerNonNull*                                                          \
 nsGenericHTMLElement::GetOn##name_()                                          \
 {                                                                             \
-  if (Tag() == nsGkAtoms::body || Tag() == nsGkAtoms::frameset) {             \
+  if (IsAnyOfHTMLElements(nsGkAtoms::body, nsGkAtoms::frameset)) {            \
     /* XXXbz note to self: add tests for this! */                             \
     nsPIDOMWindow* win = OwnerDoc()->GetInnerWindow();                        \
     if (win) {                                                                \
@@ -825,7 +828,7 @@ nsGenericHTMLElement::GetOn##name_()                                          \
 void                                                                          \
 nsGenericHTMLElement::SetOn##name_(EventHandlerNonNull* handler)              \
 {                                                                             \
-  if (Tag() == nsGkAtoms::body || Tag() == nsGkAtoms::frameset) {             \
+  if (IsAnyOfHTMLElements(nsGkAtoms::body, nsGkAtoms::frameset)) {            \
     nsPIDOMWindow* win = OwnerDoc()->GetInnerWindow();                        \
     if (!win) {                                                               \
       return;                                                                 \
@@ -842,7 +845,7 @@ nsGenericHTMLElement::SetOn##name_(EventHandlerNonNull* handler)              \
 already_AddRefed<EventHandlerNonNull>                                         \
 nsGenericHTMLElement::GetOn##name_()                                          \
 {                                                                             \
-  if (Tag() == nsGkAtoms::body || Tag() == nsGkAtoms::frameset) {             \
+  if (IsAnyOfHTMLElements(nsGkAtoms::body, nsGkAtoms::frameset)) {            \
     /* XXXbz note to self: add tests for this! */                             \
     nsPIDOMWindow* win = OwnerDoc()->GetInnerWindow();                        \
     if (win) {                                                                \
@@ -864,7 +867,7 @@ nsGenericHTMLElement::GetOn##name_()                                          \
 void                                                                          \
 nsGenericHTMLElement::SetOn##name_(EventHandlerNonNull* handler)              \
 {                                                                             \
-  if (Tag() == nsGkAtoms::body || Tag() == nsGkAtoms::frameset) {             \
+  if (IsAnyOfHTMLElements(nsGkAtoms::body, nsGkAtoms::frameset)) {            \
     nsPIDOMWindow* win = OwnerDoc()->GetInnerWindow();                        \
     if (!win) {                                                               \
       return;                                                                 \
@@ -1012,7 +1015,7 @@ nsGenericHTMLElement::ParseAttribute(int32_t aNamespaceID,
 
       aResult.ParseAtom(aValue);
 
-      if (CanHaveName(Tag())) {
+      if (CanHaveName(NodeInfo()->NameAtom())) {
         SetHasName();
         AddToNameTable(aResult.GetAtomValue());
       }
@@ -1797,17 +1800,15 @@ nsGenericHTMLElement::GetContextMenu(nsIDOMHTMLMenuElement** aContextMenu)
 bool
 nsGenericHTMLElement::IsLabelable() const
 {
-  return Tag() == nsGkAtoms::progress ||
-         Tag() == nsGkAtoms::meter;
+  return IsAnyOfHTMLElements(nsGkAtoms::progress, nsGkAtoms::meter);
 }
 
 bool
 nsGenericHTMLElement::IsInteractiveHTMLContent(bool aIgnoreTabindex) const
 {
-  return Tag() == nsGkAtoms::details ||
-         Tag() == nsGkAtoms::embed ||
-         Tag() == nsGkAtoms::keygen ||
-         (!aIgnoreTabindex && HasAttr(kNameSpaceID_None, nsGkAtoms::tabindex));
+  return IsAnyOfHTMLElements(nsGkAtoms::details, nsGkAtoms::embed,
+                             nsGkAtoms::keygen) ||
+         HasAttr(kNameSpaceID_None, nsGkAtoms::tabindex);
 }
 
 already_AddRefed<UndoManager>
@@ -2401,7 +2402,7 @@ nsGenericHTMLFormElement::FormIdUpdated(Element* aOldElement,
   nsGenericHTMLFormElement* element =
     static_cast<nsGenericHTMLFormElement*>(aData);
 
-  NS_ASSERTION(element->IsHTML(), "aData should be an HTML element");
+  NS_ASSERTION(element->IsHTMLElement(), "aData should be an HTML element");
 
   element->UpdateFormOwner(false, aNewElement);
 
@@ -2476,7 +2477,7 @@ nsGenericHTMLFormElement::UpdateFormOwner(bool aBindToTree,
                      "element should be equals to the current element "
                      "associated with the id in @form!");
 
-        if (element && element->IsHTML(nsGkAtoms::form)) {
+        if (element && element->IsHTMLElement(nsGkAtoms::form)) {
           mForm = static_cast<HTMLFormElement*>(element);
         }
       }
@@ -2814,7 +2815,7 @@ nsGenericHTMLElement::IsCurrentBodyElement()
 {
   // TODO Bug 698498: Should this handle the case where GetBody returns a
   //                  frameset?
-  if (!IsHTML(nsGkAtoms::body)) {
+  if (!IsHTMLElement(nsGkAtoms::body)) {
     return false;
   }
 
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index f26be8e7d7..5c45ef96f6 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -301,6 +301,25 @@ public:
     return rcFrame.height;
   }
 
+  // These methods are already implemented in nsIContent but we want something
+  // faster for HTMLElements ignoring the namespace checking.
+  // This is safe because we already know that we are in the HTML namespace.
+  inline bool IsHTMLElement() const
+  {
+    return true;
+  }
+
+  inline bool IsHTMLElement(nsIAtom* aTag) const
+  {
+    return mNodeInfo->Equals(aTag);
+  }
+
+  template<typename First, typename... Args>
+  inline bool IsAnyOfHTMLElements(First aFirst, Args... aArgs) const
+  {
+    return IsNodeInternal(aFirst, aArgs...);
+  }
+
 protected:
   virtual ~nsGenericHTMLElement() {}
 
@@ -930,19 +949,16 @@ public:
   static inline bool
   ShouldExposeNameAsHTMLDocumentProperty(Element* aElement)
   {
-    return aElement->IsHTML() && CanHaveName(aElement->Tag());
+    return aElement->IsHTMLElement() &&
+           CanHaveName(aElement->NodeInfo()->NameAtom());
   }
   static inline bool
   ShouldExposeIdAsHTMLDocumentProperty(Element* aElement)
   {
-    if (!aElement->IsHTML()) {
-      return false;
-    }
-    nsIAtom* tag = aElement->Tag();
-    return tag == nsGkAtoms::img ||
-           tag == nsGkAtoms::applet ||
-           tag == nsGkAtoms::embed ||
-           tag == nsGkAtoms::object;
+    return aElement->IsAnyOfHTMLElements(nsGkAtoms::img,
+                                         nsGkAtoms::applet,
+                                         nsGkAtoms::embed,
+                                         nsGkAtoms::object);
   }
 
   static bool
diff --git a/dom/html/nsGenericHTMLFrameElement.cpp b/dom/html/nsGenericHTMLFrameElement.cpp
index 4aac883301..8d7537108c 100644
--- a/dom/html/nsGenericHTMLFrameElement.cpp
+++ b/dom/html/nsGenericHTMLFrameElement.cpp
@@ -265,7 +265,7 @@ nsGenericHTMLFrameElement::SetAttr(int32_t aNameSpaceID, nsIAtom* aName,
   NS_ENSURE_SUCCESS(rv, rv);
 
   if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::src &&
-      (Tag() != nsGkAtoms::iframe || 
+      (!IsHTMLElement(nsGkAtoms::iframe) ||
        !HasAttr(kNameSpaceID_None,nsGkAtoms::srcdoc))) {
     // Don't propagate error here. The attribute was successfully set, that's
     // what we should reflect.
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index af0b06dd66..3a507f6a3c 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -684,9 +684,9 @@ nsHTMLDocument::StartDocumentLoad(const char* aCommand,
     }
   }
 
-  if (!IsHTML() || !docShell) { // no docshell for text/html XHR
-    charsetSource = IsHTML() ? kCharsetFromFallback
-                             : kCharsetFromDocTypeDefault;
+  if (!IsHTMLDocument() || !docShell) { // no docshell for text/html XHR
+    charsetSource = IsHTMLDocument() ? kCharsetFromFallback
+                                     : kCharsetFromDocTypeDefault;
     charset.AssignLiteral("UTF-8");
     TryChannelCharset(aChannel, charsetSource, charset, executor);
     parserCharsetSource = charsetSource;
@@ -778,7 +778,7 @@ nsHTMLDocument::StartDocumentLoad(const char* aCommand,
   mParser->SetDocumentCharset(parserCharset, parserCharsetSource);
   mParser->SetCommand(aCommand);
 
-  if (!IsHTML()) {
+  if (!IsHTMLDocument()) {
     MOZ_ASSERT(!loadAsHtml5);
     nsCOMPtr<nsIXMLContentSink> xmlsink;
     NS_NewXMLContentSink(getter_AddRefs(xmlsink), this, uri,
@@ -863,7 +863,7 @@ nsHTMLDocument::EndLoad()
 void
 nsHTMLDocument::SetCompatibilityMode(nsCompatibility aMode)
 {
-  NS_ASSERTION(IsHTML() || aMode == eCompatibility_FullStandards,
+  NS_ASSERTION(IsHTMLDocument() || aMode == eCompatibility_FullStandards,
                "Bad compat mode for XHTML document!");
 
   mCompatMode = aMode;
@@ -1017,7 +1017,8 @@ nsHTMLDocument::GetBody()
   for (nsIContent* child = html->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(nsGkAtoms::body) || child->IsHTML(nsGkAtoms::frameset)) {
+    if (child->IsHTMLElement(nsGkAtoms::body) ||
+        child->IsHTMLElement(nsGkAtoms::frameset)) {
       return static_cast<nsGenericHTMLElement*>(child);
     }
   }
@@ -1039,7 +1040,7 @@ NS_IMETHODIMP
 nsHTMLDocument::SetBody(nsIDOMHTMLElement* aBody)
 {
   nsCOMPtr<nsIContent> newBody = do_QueryInterface(aBody);
-  MOZ_ASSERT(!newBody || newBody->IsHTML(),
+  MOZ_ASSERT(!newBody || newBody->IsHTMLElement(),
              "How could we be an nsIContent but not actually HTML here?");
   ErrorResult rv;
   SetBody(static_cast<nsGenericHTMLElement*>(newBody.get()), rv);
@@ -1054,10 +1055,10 @@ nsHTMLDocument::SetBody(nsGenericHTMLElement* newBody, ErrorResult& rv)
   // The body element must be either a body tag or a frameset tag. And we must
   // have a html root tag, otherwise GetBody will not return the newly set
   // body.
-  if (!newBody || !(newBody->Tag() == nsGkAtoms::body ||
-                    newBody->Tag() == nsGkAtoms::frameset) ||
-      !root || !root->IsHTML() ||
-      root->Tag() != nsGkAtoms::html) {
+  if (!newBody ||
+      !newBody->IsAnyOfHTMLElements(nsGkAtoms::body, nsGkAtoms::frameset) ||
+      !root || !root->IsHTMLElement() ||
+      !root->IsHTMLElement(nsGkAtoms::html)) {
     rv.Throw(NS_ERROR_DOM_HIERARCHY_REQUEST_ERR);
     return;
   }
@@ -1420,7 +1421,7 @@ nsHTMLDocument::Open(JSContext* cx,
 {
   NS_ASSERTION(nsContentUtils::CanCallerAccess(static_cast<nsIDOMHTMLDocument*>(this)),
                "XOW should have caught this!");
-  if (!IsHTML() || mDisableDocWrite || !IsMasterDocument()) {
+  if (!IsHTMLDocument() || mDisableDocWrite || !IsMasterDocument()) {
     // No calling document.open() on XHTML
     rv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return nullptr;
@@ -1766,7 +1767,7 @@ nsHTMLDocument::Close()
 void
 nsHTMLDocument::Close(ErrorResult& rv)
 {
-  if (!IsHTML()) {
+  if (!IsHTMLDocument()) {
     // No calling document.close() on XHTML!
 
     rv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
@@ -1851,7 +1852,7 @@ nsHTMLDocument::WriteCommon(JSContext *cx,
     (mWriteLevel > NS_MAX_DOCUMENT_WRITE_DEPTH || mTooDeepWriteRecursion);
   NS_ENSURE_STATE(!mTooDeepWriteRecursion);
 
-  if (!IsHTML() || mDisableDocWrite || !IsMasterDocument()) {
+  if (!IsHTMLDocument() || mDisableDocWrite || !IsMasterDocument()) {
     // No calling document.write*() on XHTML!
 
     return NS_ERROR_DOM_INVALID_STATE_ERR;
@@ -1997,7 +1998,7 @@ nsHTMLDocument::GetElementsByName(const nsAString& aElementName,
 static bool MatchItems(nsIContent* aContent, int32_t aNameSpaceID, 
                        nsIAtom* aAtom, void* aData)
 {
-  if (!(aContent->IsElement() && aContent->AsElement()->IsHTML())) {
+  if (!aContent->IsHTMLElement()) {
     return false;
   }
 
diff --git a/dom/html/nsTextEditorState.cpp b/dom/html/nsTextEditorState.cpp
index c17174ee22..23268fada1 100644
--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -136,7 +136,7 @@ nsITextControlElement::GetWrapPropertyEnum(nsIContent* aContent,
   aWrapProp = eHTMLTextWrap_Soft; // the default
 
   nsAutoString wrap;
-  if (aContent->IsHTML()) {
+  if (aContent->IsHTMLElement()) {
     static nsIContent::AttrValuesArray strings[] =
       {&nsGkAtoms::HARD, &nsGkAtoms::OFF, nullptr};
 
@@ -570,7 +570,7 @@ nsTextInputSelectionImpl::CompleteMove(bool aForward, bool aExtend)
     {
       nsIContent *child = parentDIV->GetLastChild();
 
-      if (child->Tag() == nsGkAtoms::br)
+      if (child->IsHTMLElement(nsGkAtoms::br))
       {
         --offset;
         hint = CARET_ASSOCIATE_AFTER; // for Bug 106855
diff --git a/dom/mathml/nsMathMLElement.cpp b/dom/mathml/nsMathMLElement.cpp
index 84ffdf4da0..f45c0ea1c5 100644
--- a/dom/mathml/nsMathMLElement.cpp
+++ b/dom/mathml/nsMathMLElement.cpp
@@ -150,7 +150,7 @@ nsMathMLElement::ParseAttribute(int32_t aNamespaceID,
                                 nsAttrValue& aResult)
 {
   if (aNamespaceID == kNameSpaceID_None) {
-    if (Tag() == nsGkAtoms::math && aAttribute == nsGkAtoms::mode) {
+    if (IsMathMLElement(nsGkAtoms::math) && aAttribute == nsGkAtoms::mode) {
       WarnDeprecated(nsGkAtoms::mode->GetUTF16String(),
                      nsGkAtoms::display->GetUTF16String(), OwnerDoc());
     }
@@ -232,42 +232,40 @@ nsMathMLElement::IsAttributeMapped(const nsIAtom* aAttribute) const
   };
 
   // We don't support mglyph (yet).
-  nsIAtom* tag = Tag();
-  if (tag == nsGkAtoms::ms_ || tag == nsGkAtoms::mi_ ||
-      tag == nsGkAtoms::mn_ || tag == nsGkAtoms::mo_ ||
-      tag == nsGkAtoms::mtext_ || tag == nsGkAtoms::mspace_)
+  if (IsAnyOfMathMLElements(nsGkAtoms::ms_, nsGkAtoms::mi_, nsGkAtoms::mn_,
+                            nsGkAtoms::mo_, nsGkAtoms::mtext_,
+                            nsGkAtoms::mspace_))
     return FindAttributeDependence(aAttribute, tokenMap);
-  if (tag == nsGkAtoms::mstyle_ ||
-      tag == nsGkAtoms::math)
+  if (IsAnyOfMathMLElements(nsGkAtoms::mstyle_, nsGkAtoms::math))
     return FindAttributeDependence(aAttribute, mstyleMap);
 
-  if (tag == nsGkAtoms::mtable_)
+  if (IsMathMLElement(nsGkAtoms::mtable_))
     return FindAttributeDependence(aAttribute, mtableMap);
 
-  if (tag == nsGkAtoms::mrow_)
+  if (IsMathMLElement(nsGkAtoms::mrow_))
     return FindAttributeDependence(aAttribute, mrowMap);
 
-  if (tag == nsGkAtoms::maction_ ||
-      tag == nsGkAtoms::maligngroup_ ||
-      tag == nsGkAtoms::malignmark_ ||
-      tag == nsGkAtoms::menclose_ ||
-      tag == nsGkAtoms::merror_ ||
-      tag == nsGkAtoms::mfenced_ ||
-      tag == nsGkAtoms::mfrac_ ||
-      tag == nsGkAtoms::mover_ ||
-      tag == nsGkAtoms::mpadded_ ||
-      tag == nsGkAtoms::mphantom_ ||
-      tag == nsGkAtoms::mprescripts_ ||
-      tag == nsGkAtoms::mroot_ ||
-      tag == nsGkAtoms::msqrt_ ||
-      tag == nsGkAtoms::msub_ ||
-      tag == nsGkAtoms::msubsup_ ||
-      tag == nsGkAtoms::msup_ ||
-      tag == nsGkAtoms::mtd_ ||
-      tag == nsGkAtoms::mtr_ ||
-      tag == nsGkAtoms::munder_ ||
-      tag == nsGkAtoms::munderover_ ||
-      tag == nsGkAtoms::none) {
+  if (IsAnyOfMathMLElements(nsGkAtoms::maction_,
+                            nsGkAtoms::maligngroup_,
+                            nsGkAtoms::malignmark_,
+                            nsGkAtoms::menclose_,
+                            nsGkAtoms::merror_,
+                            nsGkAtoms::mfenced_,
+                            nsGkAtoms::mfrac_,
+                            nsGkAtoms::mover_,
+                            nsGkAtoms::mpadded_,
+                            nsGkAtoms::mphantom_,
+                            nsGkAtoms::mprescripts_,
+                            nsGkAtoms::mroot_,
+                            nsGkAtoms::msqrt_,
+                            nsGkAtoms::msub_,
+                            nsGkAtoms::msubsup_,
+                            nsGkAtoms::msup_,
+                            nsGkAtoms::mtd_,
+                            nsGkAtoms::mtr_,
+                            nsGkAtoms::munder_,
+                            nsGkAtoms::munderover_,
+                            nsGkAtoms::none)) {
     return FindAttributeDependence(aAttribute, commonPresMap);
   }
 
@@ -972,11 +970,8 @@ nsMathMLElement::IsLink(nsIURI** aURI) const
 {
   // http://www.w3.org/TR/2010/REC-MathML3-20101021/chapter6.html#interf.link
   // The REC says that the following elements should not be linking elements:
-  nsIAtom* tag = Tag();
-  if (tag == nsGkAtoms::mprescripts_ ||
-      tag == nsGkAtoms::none         ||
-      tag == nsGkAtoms::malignmark_  ||
-      tag == nsGkAtoms::maligngroup_) {
+  if (IsAnyOfMathMLElements(nsGkAtoms::mprescripts_, nsGkAtoms::none,
+                            nsGkAtoms::malignmark_, nsGkAtoms::maligngroup_)) {
     *aURI = nullptr;
     return false;
   }
diff --git a/dom/plugins/base/nsPluginInstanceOwner.cpp b/dom/plugins/base/nsPluginInstanceOwner.cpp
index 426d9bde3c..0c70a5862d 100644
--- a/dom/plugins/base/nsPluginInstanceOwner.cpp
+++ b/dom/plugins/base/nsPluginInstanceOwner.cpp
@@ -1004,13 +1004,11 @@ NS_IMETHODIMP nsPluginInstanceOwner::GetTagType(nsPluginTagType *result)
   *result = nsPluginTagType_Unknown;
 
   nsCOMPtr<nsIContent> content = do_QueryReferent(mContent);
-  nsIAtom *atom = content->Tag();
-
-  if (atom == nsGkAtoms::applet)
+  if (content->IsHTMLElement(nsGkAtoms::applet))
     *result = nsPluginTagType_Applet;
-  else if (atom == nsGkAtoms::embed)
+  else if (content->IsHTMLElement(nsGkAtoms::embed))
     *result = nsPluginTagType_Embed;
-  else if (atom == nsGkAtoms::object)
+  else if (content->IsHTMLElement(nsGkAtoms::object))
     *result = nsPluginTagType_Object;
 
   return NS_OK;
diff --git a/dom/smil/nsSMILAnimationController.cpp b/dom/smil/nsSMILAnimationController.cpp
index 93bd666325..44ac63b66b 100644
--- a/dom/smil/nsSMILAnimationController.cpp
+++ b/dom/smil/nsSMILAnimationController.cpp
@@ -780,7 +780,7 @@ nsSMILAnimationController::GetTargetIdentifierForAnimation(
   // animateTransform can only animate transforms, conversely transforms
   // can only be animated by animateTransform
   if (IsTransformAttribute(attributeNamespaceID, attributeName) !=
-      (aAnimElem->Tag() == nsGkAtoms::animateTransform))
+      (aAnimElem->IsSVGElement(nsGkAtoms::animateTransform)))
     return false;
 
   // Look up target (animated) attribute-type
diff --git a/dom/svg/SVGAnimationElement.cpp b/dom/svg/SVGAnimationElement.cpp
index eabb9227dc..b111a45948 100644
--- a/dom/svg/SVGAnimationElement.cpp
+++ b/dom/svg/SVGAnimationElement.cpp
@@ -138,7 +138,8 @@ SVGAnimationElement::GetTargetElement()
   // We'll just call the other GetTargetElement method, and QI to the right type
   nsIContent* target = GetTargetElementContent();
 
-  return (target && target->IsSVG()) ? static_cast<nsSVGElement*>(target) : nullptr;
+  return (target && target->IsSVGElement())
+           ? static_cast<nsSVGElement*>(target) : nullptr;
 }
 
 float
diff --git a/dom/svg/SVGContentUtils.cpp b/dom/svg/SVGContentUtils.cpp
index 1af5bc0e92..6d88c51d1b 100644
--- a/dom/svg/SVGContentUtils.cpp
+++ b/dom/svg/SVGContentUtils.cpp
@@ -41,13 +41,13 @@ SVGContentUtils::GetOuterSVGElement(nsSVGElement *aSVGElement)
   nsIContent *element = nullptr;
   nsIContent *ancestor = aSVGElement->GetFlattenedTreeParent();
 
-  while (ancestor && ancestor->IsSVG() &&
-                     ancestor->Tag() != nsGkAtoms::foreignObject) {
+  while (ancestor && ancestor->IsSVGElement() &&
+                     !ancestor->IsSVGElement(nsGkAtoms::foreignObject)) {
     element = ancestor;
     ancestor = element->GetFlattenedTreeParent();
   }
 
-  if (element && element->Tag() == nsGkAtoms::svg) {
+  if (element && element->IsSVGElement(nsGkAtoms::svg)) {
     return static_cast<SVGSVGElement*>(element);
   }
   return nullptr;
@@ -101,7 +101,7 @@ GetStrokeDashData(SVGContentUtils::AutoStrokeOptions* aStrokeOptions,
       return eContinuousStroke;
     }
     Float pathScale = 1.0;
-    if (aElement->Tag() == nsGkAtoms::path) {
+    if (aElement->IsSVGElement(nsGkAtoms::path)) {
       pathScale = static_cast<SVGPathElement*>(aElement)->
         GetPathLengthScale(SVGPathElement::eForStroking);
       if (pathScale <= 0) {
@@ -362,10 +362,9 @@ SVGContentUtils::EstablishesViewport(nsIContent *aContent)
   // Although SVG 1.1 states that <image> is an element that establishes a
   // viewport, this is really only for the document it references, not
   // for any child content, which is what this function is used for.
-  return aContent && aContent->IsSVG() &&
-           (aContent->Tag() == nsGkAtoms::svg ||
-            aContent->Tag() == nsGkAtoms::foreignObject ||
-            aContent->Tag() == nsGkAtoms::symbol);
+  return aContent && aContent->IsAnyOfSVGElements(nsGkAtoms::svg,
+                                                  nsGkAtoms::foreignObject,
+                                                  nsGkAtoms::symbol);
 }
 
 nsSVGElement*
@@ -373,9 +372,9 @@ SVGContentUtils::GetNearestViewportElement(nsIContent *aContent)
 {
   nsIContent *element = aContent->GetFlattenedTreeParent();
 
-  while (element && element->IsSVG()) {
+  while (element && element->IsSVGElement()) {
     if (EstablishesViewport(element)) {
-      if (element->Tag() == nsGkAtoms::foreignObject) {
+      if (element->IsSVGElement(nsGkAtoms::foreignObject)) {
         return nullptr;
       }
       return static_cast<nsSVGElement*>(element);
@@ -393,8 +392,8 @@ GetCTMInternal(nsSVGElement *aElement, bool aScreenCTM, bool aHaveRecursed)
   nsSVGElement *element = aElement;
   nsIContent *ancestor = aElement->GetFlattenedTreeParent();
 
-  while (ancestor && ancestor->IsSVG() &&
-                     ancestor->Tag() != nsGkAtoms::foreignObject) {
+  while (ancestor && ancestor->IsSVGElement() &&
+                     !ancestor->IsSVGElement(nsGkAtoms::foreignObject)) {
     element = static_cast<nsSVGElement*>(ancestor);
     matrix *= element->PrependLocalTransformsTo(gfxMatrix()); // i.e. *A*ppend
     if (!aScreenCTM && SVGContentUtils::EstablishesViewport(element)) {
@@ -412,7 +411,7 @@ GetCTMInternal(nsSVGElement *aElement, bool aScreenCTM, bool aHaveRecursed)
     // didn't find a nearestViewportElement
     return gfx::Matrix(0.0, 0.0, 0.0, 0.0, 0.0, 0.0); // singular
   }
-  if (element->Tag() != nsGkAtoms::svg) {
+  if (!element->IsSVGElement(nsGkAtoms::svg)) {
     // Not a valid SVG fragment
     return gfx::Matrix(0.0, 0.0, 0.0, 0.0, 0.0, 0.0); // singular
   }
@@ -428,7 +427,7 @@ GetCTMInternal(nsSVGElement *aElement, bool aScreenCTM, bool aHaveRecursed)
   if (!ancestor || !ancestor->IsElement()) {
     return gfx::ToMatrix(matrix);
   }
-  if (ancestor->IsSVG()) {
+  if (ancestor->IsSVGElement()) {
     return
       gfx::ToMatrix(matrix) * GetCTMInternal(static_cast<nsSVGElement*>(ancestor), true, true);
   }
diff --git a/dom/svg/SVGDocument.cpp b/dom/svg/SVGDocument.cpp
index 715c71916e..8ad666bc64 100644
--- a/dom/svg/SVGDocument.cpp
+++ b/dom/svg/SVGDocument.cpp
@@ -58,7 +58,7 @@ SVGDocument::GetRootElement(ErrorResult& aRv)
   if (!root) {
     return nullptr;
   }
-  if (!root->IsSVG()) {
+  if (!root->IsSVGElement()) {
     aRv.Throw(NS_NOINTERFACE);
     return nullptr;
   }
@@ -70,7 +70,7 @@ SVGDocument::InsertChildAt(nsIContent* aKid, uint32_t aIndex, bool aNotify)
 {
   nsresult rv = XMLDocument::InsertChildAt(aKid, aIndex, aNotify);
 
-  if (NS_SUCCEEDED(rv) && aKid->IsElement() && !aKid->IsSVG()) {
+  if (NS_SUCCEEDED(rv) && aKid->IsElement() && !aKid->IsSVGElement()) {
     // We can get here when well formed XML with a non-SVG root element is
     // served with the SVG MIME type, for example. In that case we need to load
     // the non-SVG UA sheets or else we can get bugs like bug 1016145.
diff --git a/dom/svg/SVGFEImageElement.cpp b/dom/svg/SVGFEImageElement.cpp
index 4acbe850e9..d37f575724 100644
--- a/dom/svg/SVGFEImageElement.cpp
+++ b/dom/svg/SVGFEImageElement.cpp
@@ -353,7 +353,7 @@ SVGFEImageElement::Notify(imgIRequest* aRequest, int32_t aType, const nsIntRect*
 void
 SVGFEImageElement::Invalidate()
 {
-  if (GetParent() && GetParent()->IsSVG(nsGkAtoms::filter)) {
+  if (GetParent() && GetParent()->IsSVGElement(nsGkAtoms::filter)) {
     static_cast<SVGFilterElement*>(GetParent())->Invalidate();
   }
 }
diff --git a/dom/svg/SVGFEMergeElement.cpp b/dom/svg/SVGFEMergeElement.cpp
index 8eddd92f73..fd10849101 100644
--- a/dom/svg/SVGFEMergeElement.cpp
+++ b/dom/svg/SVGFEMergeElement.cpp
@@ -42,7 +42,7 @@ SVGFEMergeElement::GetSourceImageNames(nsTArray<nsSVGStringInfo>& aSources)
   for (nsIContent* child = nsINode::GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsSVG(nsGkAtoms::feMergeNode)) {
+    if (child->IsSVGElement(nsGkAtoms::feMergeNode)) {
       SVGFEMergeNodeElement* node = static_cast<SVGFEMergeNodeElement*>(child);
       aSources.AppendElement(nsSVGStringInfo(node->GetIn1(), node));
     }
diff --git a/dom/svg/SVGForeignObjectElement.cpp b/dom/svg/SVGForeignObjectElement.cpp
index cf0ea17019..2fb44926c5 100644
--- a/dom/svg/SVGForeignObjectElement.cpp
+++ b/dom/svg/SVGForeignObjectElement.cpp
@@ -117,7 +117,7 @@ SVGForeignObjectElement::BindToTree(nsIDocument* aDocument,
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsIDocument* doc = GetComposedDoc();
-  if (doc && doc->IsSVG()) {
+  if (doc && doc->IsSVGDocument()) {
     // We assume that we're going to have HTML content, so we ensure that the
     // UA style sheets that nsDocumentViewer::CreateStyleSet skipped when
     // it saw the document was an SVG document are loaded.
diff --git a/dom/svg/SVGFragmentIdentifier.cpp b/dom/svg/SVGFragmentIdentifier.cpp
index 874e0f8145..d47f8acfcd 100644
--- a/dom/svg/SVGFragmentIdentifier.cpp
+++ b/dom/svg/SVGFragmentIdentifier.cpp
@@ -33,7 +33,7 @@ static dom::SVGViewElement*
 GetViewElement(nsIDocument* aDocument, const nsAString& aId)
 {
   dom::Element* element = aDocument->GetElementById(aId);
-  return (element && element->IsSVG(nsGkAtoms::view)) ?
+  return (element && element->IsSVGElement(nsGkAtoms::view)) ?
             static_cast<dom::SVGViewElement*>(element) : nullptr;
 }
 
@@ -238,7 +238,7 @@ bool
 SVGFragmentIdentifier::ProcessFragmentIdentifier(nsIDocument* aDocument,
                                                  const nsAString& aAnchorName)
 {
-  MOZ_ASSERT(aDocument->GetRootElement()->IsSVG(nsGkAtoms::svg),
+  MOZ_ASSERT(aDocument->GetRootElement()->IsSVGElement(nsGkAtoms::svg),
              "expecting an SVG root element");
 
   dom::SVGSVGElement* rootElement =
diff --git a/dom/svg/SVGMPathElement.cpp b/dom/svg/SVGMPathElement.cpp
index b303176894..ebadc69c9e 100644
--- a/dom/svg/SVGMPathElement.cpp
+++ b/dom/svg/SVGMPathElement.cpp
@@ -187,7 +187,7 @@ SVGMPathElement::GetReferencedPath()
   }
 
   nsIContent* genericTarget = mHrefTarget.get();
-  if (genericTarget && genericTarget->IsSVG(nsGkAtoms::path)) {
+  if (genericTarget && genericTarget->IsSVGElement(nsGkAtoms::path)) {
     return static_cast<SVGPathElement*>(genericTarget);
   }
   return nullptr;
@@ -246,7 +246,7 @@ SVGMPathElement::UnlinkHrefTarget(bool aNotifyParent)
 void
 SVGMPathElement::NotifyParentOfMpathChange(nsIContent* aParent)
 {
-  if (aParent && aParent->IsSVG(nsGkAtoms::animateMotion)) {
+  if (aParent && aParent->IsSVGElement(nsGkAtoms::animateMotion)) {
 
     SVGAnimateMotionElement* animateMotionParent =
       static_cast<SVGAnimateMotionElement*>(aParent);
diff --git a/dom/svg/SVGMotionSMILAnimationFunction.cpp b/dom/svg/SVGMotionSMILAnimationFunction.cpp
index 3f4630a062..f55e7770a4 100644
--- a/dom/svg/SVGMotionSMILAnimationFunction.cpp
+++ b/dom/svg/SVGMotionSMILAnimationFunction.cpp
@@ -134,7 +134,7 @@ GetFirstMPathChild(nsIContent* aElem)
   for (nsIContent* child = aElem->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsSVG(nsGkAtoms::mpath)) {
+    if (child->IsSVGElement(nsGkAtoms::mpath)) {
       return static_cast<SVGMPathElement*>(child);
     }
   }
@@ -152,7 +152,7 @@ SVGMotionSMILAnimationFunction::
   MOZ_ASSERT(mPathVertices.IsEmpty(),
              "regenerating when we already have vertices");
 
-  if (!aContextElem->IsSVG()) {
+  if (!aContextElem->IsSVGElement()) {
     NS_ERROR("Uh oh, SVG animateMotion element targeting a non-SVG node");
     return;
   }
diff --git a/dom/svg/SVGSVGElement.cpp b/dom/svg/SVGSVGElement.cpp
index 9962c63675..4316a21008 100644
--- a/dom/svg/SVGSVGElement.cpp
+++ b/dom/svg/SVGSVGElement.cpp
@@ -785,13 +785,12 @@ SVGSVGElement::WillBeOutermostSVG(nsIContent* aParent,
 {
   nsIContent* parent = aBindingParent ? aBindingParent : aParent;
 
-  while (parent && parent->IsSVG()) {
-    nsIAtom* tag = parent->Tag();
-    if (tag == nsGkAtoms::foreignObject) {
+  while (parent && parent->IsSVGElement()) {
+    if (parent->IsSVGElement(nsGkAtoms::foreignObject)) {
       // SVG in a foreignObject must have its own <svg> (nsSVGOuterSVGFrame).
       return false;
     }
-    if (tag == nsGkAtoms::svg) {
+    if (parent->IsSVGElement(nsGkAtoms::svg)) {
       return false;
     }
     parent = parent->GetParent();
@@ -826,7 +825,7 @@ SVGSVGElement::GetCurrentViewElement() const
     nsIDocument* doc = GetUncomposedDoc();
     if (doc) {
       Element *element = doc->GetElementById(*mCurrentViewID);
-      if (element && element->IsSVG(nsGkAtoms::view)) {
+      if (element && element->IsSVGElement(nsGkAtoms::view)) {
         return static_cast<SVGViewElement*>(element);
       }
     }
diff --git a/dom/svg/SVGSVGElement.h b/dom/svg/SVGSVGElement.h
index fdc9cf466f..25b324dd75 100644
--- a/dom/svg/SVGSVGElement.h
+++ b/dom/svg/SVGSVGElement.h
@@ -312,8 +312,8 @@ private:
    */
   bool IsInner() const {
     const nsIContent *parent = GetFlattenedTreeParent();
-    return parent && parent->IsSVG() &&
-           parent->Tag() != nsGkAtoms::foreignObject;
+    return parent && parent->IsSVGElement() &&
+           !parent->IsSVGElement(nsGkAtoms::foreignObject);
   }
 
   /* 
diff --git a/dom/svg/SVGUseElement.cpp b/dom/svg/SVGUseElement.cpp
index aabf3ff2ff..9ef00ca7c5 100644
--- a/dom/svg/SVGUseElement.cpp
+++ b/dom/svg/SVGUseElement.cpp
@@ -216,25 +216,24 @@ SVGUseElement::CreateAnonymousContent()
 
   LookupHref();
   nsIContent* targetContent = mSource.get();
-  if (!targetContent || !targetContent->IsSVG())
+  if (!targetContent)
     return nullptr;
 
   // make sure target is valid type for <use>
   // QIable nsSVGGraphicsElement would eliminate enumerating all elements
-  nsIAtom *tag = targetContent->Tag();
-  if (tag != nsGkAtoms::svg &&
-      tag != nsGkAtoms::symbol &&
-      tag != nsGkAtoms::g &&
-      tag != nsGkAtoms::path &&
-      tag != nsGkAtoms::text &&
-      tag != nsGkAtoms::rect &&
-      tag != nsGkAtoms::circle &&
-      tag != nsGkAtoms::ellipse &&
-      tag != nsGkAtoms::line &&
-      tag != nsGkAtoms::polyline &&
-      tag != nsGkAtoms::polygon &&
-      tag != nsGkAtoms::image &&
-      tag != nsGkAtoms::use)
+  if (!targetContent->IsAnyOfSVGElements(nsGkAtoms::svg,
+                                         nsGkAtoms::symbol,
+                                         nsGkAtoms::g,
+                                         nsGkAtoms::path,
+                                         nsGkAtoms::text,
+                                         nsGkAtoms::rect,
+                                         nsGkAtoms::circle,
+                                         nsGkAtoms::ellipse,
+                                         nsGkAtoms::line,
+                                         nsGkAtoms::polyline,
+                                         nsGkAtoms::polygon,
+                                         nsGkAtoms::image,
+                                         nsGkAtoms::use))
     return nullptr;
 
   // circular loop detection
@@ -248,7 +247,7 @@ SVGUseElement::CreateAnonymousContent()
     for (nsCOMPtr<nsIContent> content = GetParent();
          content;
          content = content->GetParent()) {
-      if (content->IsSVG(nsGkAtoms::use) &&
+      if (content->IsSVGElement(nsGkAtoms::use) &&
           static_cast<SVGUseElement*>(content.get())->mOriginal == mOriginal) {
         return nullptr;
       }
@@ -268,7 +267,7 @@ SVGUseElement::CreateAnonymousContent()
   if (!newcontent)
     return nullptr;
 
-  if (newcontent->IsSVG(nsGkAtoms::symbol)) {
+  if (newcontent->IsSVGElement(nsGkAtoms::symbol)) {
     nsIDocument *document = GetComposedDoc();
     if (!document)
       return nullptr;
@@ -312,8 +311,7 @@ SVGUseElement::CreateAnonymousContent()
     newcontent = svgNode;
   }
 
-  if (newcontent->IsSVG() && (newcontent->Tag() == nsGkAtoms::svg ||
-                              newcontent->Tag() == nsGkAtoms::symbol)) {
+  if (newcontent->IsAnyOfSVGElements(nsGkAtoms::svg, nsGkAtoms::symbol)) {
     nsSVGElement *newElement = static_cast<nsSVGElement*>(newcontent.get());
 
     if (mLengthAttributes[ATTR_WIDTH].IsExplicitlySet())
@@ -342,8 +340,7 @@ SVGUseElement::DestroyAnonymousContent()
 bool
 SVGUseElement::OurWidthAndHeightAreUsed() const
 {
-  return mClone && mClone->IsSVG() && (mClone->Tag() == nsGkAtoms::svg ||
-                                       mClone->Tag() == nsGkAtoms::symbol);
+  return mClone && mClone->IsAnyOfSVGElements(nsGkAtoms::svg, nsGkAtoms::symbol);
 }
 
 //----------------------------------------------------------------------
@@ -364,7 +361,7 @@ SVGUseElement::SyncWidthOrHeight(nsIAtom* aName)
       target->SetLength(aName, mLengthAttributes[index]);
       return;
     }
-    if (mClone->Tag() == nsGkAtoms::svg) {
+    if (mClone->IsSVGElement(nsGkAtoms::svg)) {
       // Our width/height attribute is now no longer explicitly set, so we
       // need to revert the clone's width/height to the width/height of the
       // content that's being cloned.
diff --git a/dom/svg/SVGZoomEvent.cpp b/dom/svg/SVGZoomEvent.cpp
index bc15aab060..5af0bd8f58 100644
--- a/dom/svg/SVGZoomEvent.cpp
+++ b/dom/svg/SVGZoomEvent.cpp
@@ -56,7 +56,7 @@ SVGZoomEvent::SVGZoomEvent(EventTarget* aOwner,
         // (e.g. if this event was created by calling createEvent on a
         // non-SVGDocument), then the "New" and "Previous"
         // properties will be left null which is probably what we want.
-        if (rootElement->IsSVG(nsGkAtoms::svg)) {
+        if (rootElement->IsSVGElement(nsGkAtoms::svg)) {
           SVGSVGElement *SVGSVGElem =
             static_cast<SVGSVGElement*>(rootElement);
 
diff --git a/dom/svg/nsSVGElement.cpp b/dom/svg/nsSVGElement.cpp
index 60060ae801..2aa1ad2e54 100644
--- a/dom/svg/nsSVGElement.cpp
+++ b/dom/svg/nsSVGElement.cpp
@@ -1249,7 +1249,7 @@ nsSVGElement::UpdateContentStyleRule()
       continue; // xml:lang has precedence
     }
 
-    if (Tag() == nsGkAtoms::svg) {
+    if (IsSVGElement(nsGkAtoms::svg)) {
       // Special case: we don't want <svg> 'width'/'height' mapped into style
       // if the attribute value isn't a valid <length> according to SVG (which
       // only supports a subset of the CSS <length> values). We don't enforce
@@ -1516,12 +1516,11 @@ nsSVGElement::GetCtx() const
 {
   nsIContent* ancestor = GetFlattenedTreeParent();
 
-  while (ancestor && ancestor->IsSVG()) {
-    nsIAtom* tag = ancestor->Tag();
-    if (tag == nsGkAtoms::foreignObject) {
+  while (ancestor && ancestor->IsSVGElement()) {
+    if (ancestor->IsSVGElement(nsGkAtoms::foreignObject)) {
       return nullptr;
     }
-    if (tag == nsGkAtoms::svg) {
+    if (ancestor->IsSVGElement(nsGkAtoms::svg)) {
       return static_cast<SVGSVGElement*>(ancestor);
     }
     ancestor = ancestor->GetFlattenedTreeParent();
diff --git a/dom/svg/nsSVGFilters.cpp b/dom/svg/nsSVGFilters.cpp
index 226e92a30a..dc7237ce90 100644
--- a/dom/svg/nsSVGFilters.cpp
+++ b/dom/svg/nsSVGFilters.cpp
@@ -489,9 +489,9 @@ nsSVGFELightingElement::ComputeLightAttributes(nsSVGFilterInstance* aInstance)
   for (nsCOMPtr<nsIContent> child = nsINode::GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsSVG(nsGkAtoms::feDistantLight) ||
-        child->IsSVG(nsGkAtoms::fePointLight) ||
-        child->IsSVG(nsGkAtoms::feSpotLight)) {
+    if (child->IsAnyOfSVGElements(nsGkAtoms::feDistantLight,
+                                  nsGkAtoms::fePointLight,
+                                  nsGkAtoms::feSpotLight)) {
       return static_cast<SVGFELightElement*>(child.get())->ComputeLightAttributes(aInstance);
     }
   }
diff --git a/dom/svg/nsSVGLength2.cpp b/dom/svg/nsSVGLength2.cpp
index 2bc5083c11..a72248074f 100644
--- a/dom/svg/nsSVGLength2.cpp
+++ b/dom/svg/nsSVGLength2.cpp
@@ -227,7 +227,7 @@ float
 nsSVGLength2::GetUnitScaleFactor(nsIFrame *aFrame, uint8_t aUnitType) const
 {
   nsIContent* content = aFrame->GetContent();
-  if (content->IsSVG()) {
+  if (content->IsSVGElement()) {
     return GetUnitScaleFactor(SVGElementMetrics(static_cast<nsSVGElement*>(content)), aUnitType);
   }
   return GetUnitScaleFactor(NonSVGFrameUserSpaceMetrics(aFrame), aUnitType);
diff --git a/dom/xbl/XBLChildrenElement.h b/dom/xbl/XBLChildrenElement.h
index 5bf9ea4220..b7d7f532e9 100644
--- a/dom/xbl/XBLChildrenElement.h
+++ b/dom/xbl/XBLChildrenElement.h
@@ -133,7 +133,7 @@ public:
   {
     NS_ASSERTION(!mIncludes.IsEmpty(),
                  "Shouldn't check for includes on default insertion point");
-    return mIncludes.Contains(aChild->Tag());
+    return mIncludes.Contains(aChild->NodeInfo()->NameAtom());
   }
 
   bool IsDefaultInsertion()
diff --git a/dom/xbl/nsBindingManager.cpp b/dom/xbl/nsBindingManager.cpp
index fcb7b30212..6ef1841c0c 100644
--- a/dom/xbl/nsBindingManager.cpp
+++ b/dom/xbl/nsBindingManager.cpp
@@ -250,7 +250,7 @@ nsBindingManager::ResolveTag(nsIContent* aContent, int32_t* aNameSpaceID)
   }
 
   *aNameSpaceID = aContent->GetNameSpaceID();
-  return aContent->Tag();
+  return aContent->NodeInfo()->NameAtom();
 }
 
 nsresult
diff --git a/dom/xbl/nsXBLContentSink.cpp b/dom/xbl/nsXBLContentSink.cpp
index 422b5680c6..aa6c9f4d13 100644
--- a/dom/xbl/nsXBLContentSink.cpp
+++ b/dom/xbl/nsXBLContentSink.cpp
@@ -137,9 +137,9 @@ nsXBLContentSink::FlushText(bool aReleaseTextNode)
     nsIContent* content = GetCurrentContent();
     if (content &&
         (content->NodeInfo()->NamespaceEquals(kNameSpaceID_XBL) ||
-         (content->NodeInfo()->NamespaceEquals(kNameSpaceID_XUL) &&
-          content->Tag() != nsGkAtoms::label &&
-          content->Tag() != nsGkAtoms::description))) {
+         (content->IsXULElement() &&
+          !content->IsAnyOfXULElements(nsGkAtoms::label,
+                                       nsGkAtoms::description)))) {
 
       bool isWS = true;
       if (mTextLength > 0) {
@@ -884,7 +884,7 @@ nsresult
 nsXBLContentSink::AddAttributes(const char16_t** aAtts,
                                 nsIContent* aContent)
 {
-  if (aContent->IsXUL())
+  if (aContent->IsXULElement())
     return NS_OK; // Nothing to do, since the proto already has the attrs.
 
   return nsXMLContentSink::AddAttributes(aAtts, aContent);
diff --git a/dom/xbl/nsXBLPrototypeBinding.cpp b/dom/xbl/nsXBLPrototypeBinding.cpp
index 9e090ab252..e852a905a9 100644
--- a/dom/xbl/nsXBLPrototypeBinding.cpp
+++ b/dom/xbl/nsXBLPrototypeBinding.cpp
@@ -1480,7 +1480,7 @@ nsXBLPrototypeBinding::WriteContentNode(nsIObjectOutputStream* aStream,
   rv = aStream->WriteWStringZ(prefixStr.get());
   NS_ENSURE_SUCCESS(rv, rv);
 
-  rv = aStream->WriteWStringZ(nsDependentAtomString(aNode->Tag()).get());
+  rv = aStream->WriteWStringZ(nsDependentAtomString(aNode->NodeInfo()->NameAtom()).get());
   NS_ENSURE_SUCCESS(rv, rv);
 
   // Write attributes
diff --git a/dom/xbl/nsXBLPrototypeHandler.cpp b/dom/xbl/nsXBLPrototypeHandler.cpp
index 54c2475119..7ad1be4250 100644
--- a/dom/xbl/nsXBLPrototypeHandler.cpp
+++ b/dom/xbl/nsXBLPrototypeHandler.cpp
@@ -484,7 +484,7 @@ nsXBLPrototypeHandler::DispatchXBLCommand(EventTarget* aTarget, nsIDOMEvent* aEv
     // element is focused and has a parent.
     if (focusedContent && focusedContent->GetParent()) {
       while (content) {
-        if (content->Tag() == nsGkAtoms::a && content->IsHTML()) {
+        if (content->IsHTMLElement(nsGkAtoms::a)) {
           isLink = true;
           break;
         }
diff --git a/dom/xbl/nsXBLService.cpp b/dom/xbl/nsXBLService.cpp
index 978c6fc5da..d4dc11abbb 100644
--- a/dom/xbl/nsXBLService.cpp
+++ b/dom/xbl/nsXBLService.cpp
@@ -666,7 +666,8 @@ MayBindToContent(nsXBLPrototypeBinding* aProtoBinding, nsIContent* aBoundElement
 
   // We let XUL content and content in XUL documents through, since XUL is
   // restricted anyway and we want to minimize remote XUL breakage.
-  if (aBoundElement->IsXUL() || aBoundElement->OwnerDoc()->IsXUL()) {
+  if (aBoundElement->IsXULElement() ||
+      aBoundElement->OwnerDoc()->IsXULElement()) {
     return true;
   }
 
@@ -957,7 +958,7 @@ nsXBLService::LoadBindingDocumentInfo(nsIContent* aBoundElement,
                   ni->Equals(nsGkAtoms::thumb, kNameSpaceID_XUL) ||
                   ((ni->Equals(nsGkAtoms::input) ||
                     ni->Equals(nsGkAtoms::select)) &&
-                   aBoundElement->IsHTML()))) && !aForceSyncLoad) {
+                   aBoundElement->IsHTMLElement()))) && !aForceSyncLoad) {
       // The third line of defense is to investigate whether or not the
       // document is currently being loaded asynchronously.  If so, there's no
       // document yet, but we need to glom on our request so that it will be
diff --git a/dom/xml/nsXMLContentSink.cpp b/dom/xml/nsXMLContentSink.cpp
index 2e34df0e4d..fa782e9e60 100644
--- a/dom/xml/nsXMLContentSink.cpp
+++ b/dom/xml/nsXMLContentSink.cpp
@@ -849,7 +849,7 @@ nsXMLContentSink::PushContent(nsIContent *aContent)
 
   // When an XML parser would append a node to a template element, it
   // must instead append it to the template element's template contents.
-  if (contentToPush->IsHTML(nsGkAtoms::_template)) {
+  if (contentToPush->IsHTMLElement(nsGkAtoms::_template)) {
     HTMLTemplateElement* templateElement =
       static_cast<HTMLTemplateElement*>(contentToPush);
     contentToPush = templateElement->Content();
@@ -1116,7 +1116,7 @@ nsXMLContentSink::HandleEndElement(const char16_t *aName,
   }
   DidAddContent();
 
-  if (content->IsSVG(nsGkAtoms::svg)) {
+  if (content->IsSVGElement(nsGkAtoms::svg)) {
     FlushTags();
     nsCOMPtr<nsIRunnable> event = new nsHtml5SVGLoadDispatcher(content);
     if (NS_FAILED(NS_DispatchToMainThread(event))) {
diff --git a/dom/xml/nsXMLFragmentContentSink.cpp b/dom/xml/nsXMLFragmentContentSink.cpp
index d0862330a3..4535a670e7 100644
--- a/dom/xml/nsXMLFragmentContentSink.cpp
+++ b/dom/xml/nsXMLFragmentContentSink.cpp
@@ -228,8 +228,9 @@ nsresult
 nsXMLFragmentContentSink::CloseElement(nsIContent* aContent)
 {
   // don't do fancy stuff in nsXMLContentSink
-  if (mPreventScriptExecution && aContent->Tag() == nsGkAtoms::script &&
-      (aContent->IsHTML() || aContent->IsSVG())) {
+  if (mPreventScriptExecution &&
+      (aContent->IsHTMLElement(nsGkAtoms::script) ||
+       aContent->IsSVGElement(nsGkAtoms::script))) {
     nsCOMPtr<nsIScriptElement> sele = do_QueryInterface(aContent);
     NS_ASSERTION(sele, "script did QI correctly!");
     sele->PreventExecution();
diff --git a/dom/xslt/xpath/XPathEvaluator.cpp b/dom/xslt/xpath/XPathEvaluator.cpp
index c0edea17c9..c3508a9367 100644
--- a/dom/xslt/xpath/XPathEvaluator.cpp
+++ b/dom/xslt/xpath/XPathEvaluator.cpp
@@ -121,7 +121,8 @@ XPathEvaluator::CreateExpression(const nsAString& aExpression,
                                  XPathNSResolver* aResolver, ErrorResult& aRv)
 {
     nsCOMPtr<nsIDocument> doc = do_QueryReferent(mDocument);
-    XPathEvaluatorParseContext pContext(aResolver, !(doc && doc->IsHTML()));
+    XPathEvaluatorParseContext pContext(aResolver,
+                                        !(doc && doc->IsHTMLDocument()));
     return CreateExpression(aExpression, &pContext, doc, aRv);
 }
 
@@ -130,7 +131,8 @@ XPathEvaluator::CreateExpression(const nsAString& aExpression,
                                  nsINode* aResolver, ErrorResult& aRv)
 {
     nsCOMPtr<nsIDocument> doc = do_QueryReferent(mDocument);
-    XPathEvaluatorParseContext pContext(aResolver, !(doc && doc->IsHTML()));
+    XPathEvaluatorParseContext pContext(aResolver,
+                                        !(doc && doc->IsHTMLDocument()));
     return CreateExpression(aExpression, &pContext, doc, aRv);
 }
 
diff --git a/dom/xslt/xpath/txMozillaXPathTreeWalker.cpp b/dom/xslt/xpath/txMozillaXPathTreeWalker.cpp
index ad18c4fb2d..c0338ec0e6 100644
--- a/dom/xslt/xpath/txMozillaXPathTreeWalker.cpp
+++ b/dom/xslt/xpath/txMozillaXPathTreeWalker.cpp
@@ -348,7 +348,8 @@ txXPathNodeUtils::getLocalName(const txXPathNode& aNode)
 
     if (aNode.isContent()) {
         if (aNode.mNode->IsElement()) {
-            nsCOMPtr<nsIAtom> localName = aNode.Content()->Tag();
+            nsCOMPtr<nsIAtom> localName =
+                aNode.Content()->NodeInfo()->NameAtom();
             return localName.forget();
         }
 
@@ -420,7 +421,7 @@ txXPathNodeUtils::getLocalName(const txXPathNode& aNode, nsAString& aLocalName)
 
     // Check for html
     if (aNode.Content()->NodeInfo()->NamespaceEquals(kNameSpaceID_None) &&
-        aNode.Content()->IsHTML()) {
+        aNode.Content()->IsHTMLElement()) {
         nsContentUtils::ASCIIToUpper(aLocalName);
     }
 }
diff --git a/dom/xslt/xpath/txXPathTreeWalker.h b/dom/xslt/xpath/txXPathTreeWalker.h
index 2cb3ac5a3f..26cb42ddd4 100644
--- a/dom/xslt/xpath/txXPathTreeWalker.h
+++ b/dom/xslt/xpath/txXPathTreeWalker.h
@@ -110,7 +110,7 @@ public:
         return false;
       }
       nsIContent* content = aNode.Content();
-      return content->IsHTML() && content->IsInHTMLDocument();
+      return content->IsHTMLElement() && content->IsInHTMLDocument();
     }
 };
 
diff --git a/dom/xslt/xslt/txMozillaXMLOutput.cpp b/dom/xslt/xslt/txMozillaXMLOutput.cpp
index 954213098d..03a3c58e35 100644
--- a/dom/xslt/xslt/txMozillaXMLOutput.cpp
+++ b/dom/xslt/xslt/txMozillaXMLOutput.cpp
@@ -281,24 +281,21 @@ txMozillaXMLOutput::endElement()
 
     // Handle html-elements
     if (!mNoFixup) {
-        if (element->IsHTML()) {
+        if (element->IsHTMLElement()) {
             rv = endHTMLElement(element);
             NS_ENSURE_SUCCESS(rv, rv);
         }
 
         // Handle elements that are different when parser-created
-        int32_t ns = element->GetNameSpaceID();
-        nsIAtom* localName = element->Tag();
-
-        if ((ns == kNameSpaceID_XHTML && (localName == nsGkAtoms::title ||
-                                          localName == nsGkAtoms::object ||
-                                          localName == nsGkAtoms::applet ||
-                                          localName == nsGkAtoms::select ||
-                                          localName == nsGkAtoms::textarea)) ||
-            (ns == kNameSpaceID_SVG && localName == nsGkAtoms::title)) {
+        if (element->IsAnyOfHTMLElements(nsGkAtoms::title,
+                                         nsGkAtoms::object,
+                                         nsGkAtoms::applet,
+                                         nsGkAtoms::select,
+                                         nsGkAtoms::textarea) ||
+            element->IsSVGElement(nsGkAtoms::title)) {
             element->DoneAddingChildren(true);
-        } else if ((ns == kNameSpaceID_XHTML || ns == kNameSpaceID_SVG) &&
-                   localName == nsGkAtoms::script) {
+        } else if (element->IsSVGElement(nsGkAtoms::script) ||
+                   element->IsHTMLElement(nsGkAtoms::script)) {
             nsCOMPtr<nsIScriptElement> sele = do_QueryInterface(element);
             MOZ_ASSERT(sele, "script elements need to implement nsIScriptElement");
             bool block = sele->AttemptToExecute();
@@ -308,12 +305,11 @@ txMozillaXMLOutput::endElement()
                 rv = mNotifier->AddScriptElement(sele);
                 NS_ENSURE_SUCCESS(rv, rv);
             }
-        } else if (ns == kNameSpaceID_XHTML &&
-                   (localName == nsGkAtoms::input ||
-                    localName == nsGkAtoms::button ||
-                    localName == nsGkAtoms::menuitem ||
-                    localName == nsGkAtoms::audio ||
-                    localName == nsGkAtoms::video)) {
+        } else if (element->IsAnyOfHTMLElements(nsGkAtoms::input,
+                                                nsGkAtoms::button,
+                                                nsGkAtoms::menuitem,
+                                                nsGkAtoms::audio,
+                                                nsGkAtoms::video)) {
           element->DoneCreatingElement();
         }   
     }
@@ -645,7 +641,7 @@ txMozillaXMLOutput::createTxWrapper()
         }
 #endif
 
-        if (childContent->Tag() == nsGkAtoms::documentTypeNodeName) {
+        if (childContent->NodeInfo()->NameAtom() == nsGkAtoms::documentTypeNodeName) {
 #ifdef DEBUG
             // The new documentElement should go after the document type.
             // This is needed for cases when there is no existing
@@ -677,9 +673,8 @@ nsresult
 txMozillaXMLOutput::startHTMLElement(nsIContent* aElement, bool aIsHTML)
 {
     nsresult rv = NS_OK;
-    nsIAtom *atom = aElement->Tag();
 
-    if ((atom != nsGkAtoms::tr || !aIsHTML) &&
+    if ((!aElement->IsHTMLElement(nsGkAtoms::tr) || !aIsHTML) &&
         NS_PTR_TO_INT32(mTableStateStack.peek()) == ADDED_TBODY) {
         uint32_t last = mCurrentNodeStack.Count() - 1;
         NS_ASSERTION(last != (uint32_t)-1, "empty stack");
@@ -689,10 +684,10 @@ txMozillaXMLOutput::startHTMLElement(nsIContent* aElement, bool aIsHTML)
         mTableStateStack.pop();
     }
 
-    if (atom == nsGkAtoms::table && aIsHTML) {
+    if (aElement->IsHTMLElement(nsGkAtoms::table) && aIsHTML) {
         mTableState = TABLE;
     }
-    else if (atom == nsGkAtoms::tr && aIsHTML &&
+    else if (aElement->IsHTMLElement(nsGkAtoms::tr) && aIsHTML &&
              NS_PTR_TO_INT32(mTableStateStack.peek()) == TABLE) {
         nsCOMPtr<nsIContent> tbody;
         rv = createHTMLElement(nsGkAtoms::tbody, getter_AddRefs(tbody));
@@ -710,7 +705,7 @@ txMozillaXMLOutput::startHTMLElement(nsIContent* aElement, bool aIsHTML)
 
         mCurrentNode = tbody;
     }
-    else if (atom == nsGkAtoms::head &&
+    else if (aElement->IsHTMLElement(nsGkAtoms::head) &&
              mOutputFormat.mMethod == eHTMLOutput) {
         // Insert META tag, according to spec, 16.2, like
         // <META http-equiv="Content-Type" content="text/html; charset=EUC-JP">
@@ -742,10 +737,8 @@ txMozillaXMLOutput::startHTMLElement(nsIContent* aElement, bool aIsHTML)
 nsresult
 txMozillaXMLOutput::endHTMLElement(nsIContent* aElement)
 {
-    nsIAtom *atom = aElement->Tag();
-
     if (mTableState == ADDED_TBODY) {
-        NS_ASSERTION(atom == nsGkAtoms::tbody,
+        NS_ASSERTION(aElement->IsHTMLElement(nsGkAtoms::tbody),
                      "Element flagged as added tbody isn't a tbody");
         uint32_t last = mCurrentNodeStack.Count() - 1;
         NS_ASSERTION(last != (uint32_t)-1, "empty stack");
@@ -757,7 +750,7 @@ txMozillaXMLOutput::endHTMLElement(nsIContent* aElement)
 
         return NS_OK;
     }
-    else if (mCreatingNewDocument && atom == nsGkAtoms::meta) {
+    else if (mCreatingNewDocument && aElement->IsHTMLElement(nsGkAtoms::meta)) {
         // handle HTTP-EQUIV data
         nsAutoString httpEquiv;
         aElement->GetAttr(kNameSpaceID_None, nsGkAtoms::httpEquiv, httpEquiv);
diff --git a/dom/xslt/xslt/txMozillaXSLTProcessor.cpp b/dom/xslt/xslt/txMozillaXSLTProcessor.cpp
index 529b8135c6..6e01ab9397 100644
--- a/dom/xslt/xslt/txMozillaXSLTProcessor.cpp
+++ b/dom/xslt/xslt/txMozillaXSLTProcessor.cpp
@@ -194,7 +194,7 @@ txToFragmentHandlerFactory::createHandlerWith(txOutputFormat* aFormat,
             NS_ASSERTION(domdoc, "unable to get ownerdocument");
             nsCOMPtr<nsIDocument> doc = do_QueryInterface(domdoc);
 
-            if (doc && doc->IsHTML()) {
+            if (doc && doc->IsHTMLDocument()) {
                 format.mMethod = eHTMLOutput;
             } else {
                 format.mMethod = eXMLOutput;
diff --git a/dom/xul/XULDocument.cpp b/dom/xul/XULDocument.cpp
index e134aa23c3..4edd69422e 100644
--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -4093,12 +4093,11 @@ XULDocument::BroadcasterHookup::~BroadcasterHookup()
 #ifdef PR_LOGGING
     if (PR_LOG_TEST(gXULLog, PR_LOG_WARNING) && !mResolved) {
         // Tell the world we failed
-        nsIAtom *tag = mObservesElement->Tag();
 
         nsAutoString broadcasterID;
         nsAutoString attribute;
 
-        if (tag == nsGkAtoms::observes) {
+        if (mObservesElement->IsXULElement(nsGkAtoms::observes)) {
             mObservesElement->GetAttr(kNameSpaceID_None, nsGkAtoms::element, broadcasterID);
             mObservesElement->GetAttr(kNameSpaceID_None, nsGkAtoms::attribute, attribute);
         }
@@ -4112,7 +4111,7 @@ XULDocument::BroadcasterHookup::~BroadcasterHookup()
         broadcasteridC.AssignWithConversion(broadcasterID);
         PR_LOG(gXULLog, PR_LOG_WARNING,
                ("xul: broadcaster hookup failed <%s attribute='%s'> to %s",
-                nsAtomCString(tag).get(),
+                nsAtomCString(mObservesElement->NodeInfo()->NameAtom()).get(),
                 attributeC.get(),
                 broadcasteridC.get()));
     }
@@ -4328,7 +4327,7 @@ XULDocument::CheckBroadcasterHookup(Element* aElement,
         broadcasteridC.AssignWithConversion(broadcasterID);
         PR_LOG(gXULLog, PR_LOG_NOTICE,
                ("xul: broadcaster hookup <%s attribute='%s'> to %s",
-                nsAtomCString(content->Tag()).get(),
+                nsAtomCString(content->NodeInfo()->NameAtom()).get(),
                 attributeC.get(),
                 broadcasteridC.get()));
     }
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index acef288a26..19fd3412d3 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -632,7 +632,7 @@ nsXULElement::PerformAccesskey(bool aKeyCausesActivation,
 {
     nsCOMPtr<nsIContent> content(this);
 
-    if (Tag() == nsGkAtoms::label) {
+    if (IsXULElement(nsGkAtoms::label)) {
         nsCOMPtr<nsIDOMElement> element;
 
         nsAutoString control;
@@ -660,13 +660,12 @@ nsXULElement::PerformAccesskey(bool aKeyCausesActivation,
     nsXULElement* elm = FromContent(content);
     if (elm) {
         // Define behavior for each type of XUL element.
-        nsIAtom *tag = content->Tag();
-        if (tag != nsGkAtoms::toolbarbutton) {
+        if (!content->IsXULElement(nsGkAtoms::toolbarbutton)) {
           nsIFocusManager* fm = nsFocusManager::GetFocusManager();
           if (fm) {
             nsCOMPtr<nsIDOMElement> element;
             // for radio buttons, focus the radiogroup instead
-            if (tag == nsGkAtoms::radio) {
+            if (content->IsXULElement(nsGkAtoms::radio)) {
               nsCOMPtr<nsIDOMXULSelectControlItemElement> controlItem(do_QueryInterface(content));
               if (controlItem) {
                 bool disabled;
@@ -685,7 +684,8 @@ nsXULElement::PerformAccesskey(bool aKeyCausesActivation,
               fm->SetFocus(element, nsIFocusManager::FLAG_BYKEY);
           }
         }
-        if (aKeyCausesActivation && tag != nsGkAtoms::textbox && tag != nsGkAtoms::menulist) {
+        if (aKeyCausesActivation &&
+            !content->IsAnyOfXULElements(nsGkAtoms::textbox, nsGkAtoms::menulist)) {
           elm->ClickWithInputSource(nsIDOMMouseEvent::MOZ_SOURCE_KEYBOARD);
         }
     }
@@ -857,7 +857,7 @@ nsXULElement::BindToTree(nsIDocument* aDocument,
     // We do this during binding, not element construction, because elements
     // can be moved from the document that creates them to another document.
 
-    if (!XULElementsRulesInMinimalXULSheet(Tag())) {
+    if (!XULElementsRulesInMinimalXULSheet(NodeInfo()->NameAtom())) {
       doc->EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::XULSheet());
       // To keep memory usage down it is important that we try and avoid
       // pulling xul.css into non-XUL documents. That should be very rare, and
@@ -1287,9 +1287,8 @@ nsresult
 nsXULElement::PreHandleEvent(EventChainPreVisitor& aVisitor)
 {
     aVisitor.mForceContentDispatch = true; //FIXME! Bug 329119
-    nsIAtom* tag = Tag();
     if (IsRootOfNativeAnonymousSubtree() &&
-        (tag == nsGkAtoms::scrollbar || tag == nsGkAtoms::scrollcorner) &&
+        (IsAnyOfXULElements(nsGkAtoms::scrollbar, nsGkAtoms::scrollcorner)) &&
         (aVisitor.mEvent->message == NS_MOUSE_CLICK ||
          aVisitor.mEvent->message == NS_MOUSE_DOUBLECLICK ||
          aVisitor.mEvent->message == NS_XUL_COMMAND ||
@@ -1305,7 +1304,7 @@ nsXULElement::PreHandleEvent(EventChainPreVisitor& aVisitor)
     if (aVisitor.mEvent->message == NS_XUL_COMMAND &&
         aVisitor.mEvent->mClass == eInputEventClass &&
         aVisitor.mEvent->originalTarget == static_cast<nsIContent*>(this) &&
-        tag != nsGkAtoms::command) {
+        !IsXULElement(nsGkAtoms::command)) {
         // Check that we really have an xul command event. That will be handled
         // in a special way.
         nsCOMPtr<nsIDOMXULCommandEvent> xulEvent =
@@ -1453,8 +1452,7 @@ nsXULElement::GetAttributeChangeHint(const nsIAtom* aAttribute,
     if (aAttribute == nsGkAtoms::value &&
         (aModType == nsIDOMMutationEvent::REMOVAL ||
          aModType == nsIDOMMutationEvent::ADDITION)) {
-      nsIAtom *tag = Tag();
-      if (tag == nsGkAtoms::label || tag == nsGkAtoms::description)
+      if (IsAnyOfXULElements(nsGkAtoms::label, nsGkAtoms::description))
         // Label and description dynamically morph between a normal
         // block and a cropping single-line XUL text frame.  If the
         // value attribute is being added or removed, then we need to
@@ -1585,10 +1583,8 @@ nsXULElement::LoadSrc()
 {
     // Allow frame loader only on objects for which a container box object
     // can be obtained.
-    nsIAtom* tag = Tag();
-    if (tag != nsGkAtoms::browser &&
-        tag != nsGkAtoms::editor &&
-        tag != nsGkAtoms::iframe) {
+    if (!IsAnyOfXULElements(nsGkAtoms::browser, nsGkAtoms::editor,
+                            nsGkAtoms::iframe)) {
         return NS_OK;
     }
     if (!IsInDoc() ||
diff --git a/dom/xul/nsXULElement.h b/dom/xul/nsXULElement.h
index 9fd196313f..289ee5a35a 100644
--- a/dom/xul/nsXULElement.h
+++ b/dom/xul/nsXULElement.h
@@ -719,11 +719,8 @@ protected:
 
     bool IsReadWriteTextElement() const
     {
-        const nsIAtom* tag = Tag();
-        return
-            GetNameSpaceID() == kNameSpaceID_XUL &&
-            (tag == nsGkAtoms::textbox || tag == nsGkAtoms::textarea) &&
-            !HasAttr(kNameSpaceID_None, nsGkAtoms::readonly);
+        return IsAnyOfXULElements(nsGkAtoms::textbox, nsGkAtoms::textarea) &&
+               !HasAttr(kNameSpaceID_None, nsGkAtoms::readonly);
     }
 
     virtual JSObject* WrapNode(JSContext *aCx) override;
diff --git a/dom/xul/nsXULPopupListener.cpp b/dom/xul/nsXULPopupListener.cpp
index 0c20e72003..213d99473e 100644
--- a/dom/xul/nsXULPopupListener.cpp
+++ b/dom/xul/nsXULPopupListener.cpp
@@ -140,8 +140,7 @@ nsXULPopupListener::HandleEvent(nsIDOMEvent* aEvent)
   if (!targetContent) {
     return NS_OK;
   }
-  if (targetContent->Tag() == nsGkAtoms::browser &&
-      targetContent->IsXUL() &&
+  if (targetContent->IsXULElement(nsGkAtoms::browser) &&
       EventStateManager::IsRemoteTarget(targetContent)) {
     return NS_OK;
   }
@@ -191,8 +190,8 @@ nsXULPopupListener::HandleEvent(nsIDOMEvent* aEvent)
   // to show, we know (guaranteed) that we're dealing with a menu or
   // submenu of an already-showing popup.  We don't need to do anything at all.
   if (!mIsContext) {
-    nsIAtom *tag = targetContent ? targetContent->Tag() : nullptr;
-    if (tag == nsGkAtoms::menu || tag == nsGkAtoms::menuitem)
+    if (targetContent &&
+        targetContent->IsAnyOfXULElements(nsGkAtoms::menu, nsGkAtoms::menuitem))
       return NS_OK;
   }
 
@@ -307,7 +306,7 @@ GetImmediateChild(nsIContent* aContent, nsIAtom *aTag)
   for (nsIContent* child = aContent->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->Tag() == aTag) {
+    if (child->IsXULElement(aTag)) {
       nsCOMPtr<nsIContent> ret = child;
       return ret.forget();
     }
diff --git a/dom/xul/templates/nsXULContentBuilder.cpp b/dom/xul/templates/nsXULContentBuilder.cpp
index 574cb317be..62ed11e550 100644
--- a/dom/xul/templates/nsXULContentBuilder.cpp
+++ b/dom/xul/templates/nsXULContentBuilder.cpp
@@ -457,9 +457,9 @@ nsXULContentBuilder::BuildContentFromTemplate(nsIContent *aTemplateNode,
 
         PR_LOG(gXULTemplateLog, PR_LOG_ALWAYS,
                ("Tags: [Template: %s  Resource: %s  Real: %s] for id %s",
-                nsAtomCString(aTemplateNode->Tag()).get(), 
-                nsAtomCString(aResourceNode->Tag()).get(),
-                nsAtomCString(aRealNode->Tag()).get(), NS_ConvertUTF16toUTF8(id).get()));
+                nsAtomCString(aTemplateNode->NodeInfo()->NameAtom()).get(),
+                nsAtomCString(aResourceNode->NodeInfo()->NameAtom()).get(),
+                nsAtomCString(aRealNode->NodeInfo()->NameAtom()).get(), NS_ConvertUTF16toUTF8(id).get()));
     }
 #endif
 
@@ -523,7 +523,7 @@ nsXULContentBuilder::BuildContentFromTemplate(nsIContent *aTemplateNode,
 
         MOZ_ASSERT_IF(isGenerationElement, tmplKid->IsElement());
 
-        nsIAtom *tag = tmplKid->Tag();
+        nsIAtom *tag = tmplKid->NodeInfo()->NameAtom();
 
 #ifdef PR_LOGGING
         if (PR_LOG_TEST(gXULTemplateLog, PR_LOG_DEBUG)) {
@@ -1049,7 +1049,7 @@ nsXULContentBuilder::CreateContainerContents(nsIContent* aElement,
         nsTemplateQuerySet* queryset = mQuerySets[r];
 
         nsIAtom* tag = queryset->GetTag();
-        if (tag && tag != aElement->Tag())
+        if (tag && tag != aElement->NodeInfo()->NameAtom())
             continue;
 
         CreateContainerContentsForQuerySet(aElement, aResult, aNotify, queryset,
@@ -1263,16 +1263,13 @@ bool
 nsXULContentBuilder::IsOpen(nsIContent* aElement)
 {
     // Determine if this is a <treeitem> or <menu> element
-    if (!aElement->IsXUL())
-        return true;
 
     // XXXhyatt Use the XBL service to obtain a base tag.
-    nsIAtom *tag = aElement->Tag();
-    if (tag == nsGkAtoms::menu ||
-        tag == nsGkAtoms::menubutton ||
-        tag == nsGkAtoms::toolbarbutton ||
-        tag == nsGkAtoms::button ||
-        tag == nsGkAtoms::treeitem)
+    if (aElement->IsAnyOfXULElements(nsGkAtoms::menu,
+                                     nsGkAtoms::menubutton,
+                                     nsGkAtoms::toolbarbutton,
+                                     nsGkAtoms::button,
+                                     nsGkAtoms::treeitem))
         return aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
                                      nsGkAtoms::_true, eCaseMatters);
     return true;
@@ -1447,7 +1444,7 @@ nsXULContentBuilder::HasGeneratedContent(nsIRDFResource* aResource,
 
     // the root resource is always acceptable
     if (aResource == rootresource) {
-        if (!aTag || mRoot->Tag() == aTag)
+        if (!aTag || mRoot->NodeInfo()->NameAtom() == aTag)
             *aGenerated = true;
     }
     else {
@@ -1473,7 +1470,7 @@ nsXULContentBuilder::HasGeneratedContent(nsIRDFResource* aResource,
                 nsTemplateMatch* match;
                 if (content == mRoot || mContentSupportMap.Get(content, &match)) {
                     // If we've got a tag, check it to ensure we're consistent.
-                    if (!aTag || content->Tag() == aTag) {
+                    if (!aTag || content->NodeInfo()->NameAtom() == aTag) {
                         *aGenerated = true;
                         return NS_OK;
                     }
diff --git a/dom/xul/templates/nsXULSortService.cpp b/dom/xul/templates/nsXULSortService.cpp
index 4afd778b11..353cb1cd6c 100644
--- a/dom/xul/templates/nsXULSortService.cpp
+++ b/dom/xul/templates/nsXULSortService.cpp
@@ -75,30 +75,26 @@ XULSortServiceImpl::SetSortColumnHints(nsIContent *content,
   for (nsIContent* child = content->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsXUL()) {
-      nsIAtom *tag = child->Tag();
-
-      if (tag == nsGkAtoms::treecols) {
-        SetSortColumnHints(child, sortResource, sortDirection);
-      } else if (tag == nsGkAtoms::treecol) {
-        nsAutoString value;
-        child->GetAttr(kNameSpaceID_None, nsGkAtoms::sort, value);
-        // also check the resource attribute for older code
-        if (value.IsEmpty())
-          child->GetAttr(kNameSpaceID_None, nsGkAtoms::resource, value);
-        if (value == sortResource) {
-          child->SetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
-                         NS_LITERAL_STRING("true"), true);
-          child->SetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection,
-                         sortDirection, true);
-          // Note: don't break out of loop; want to set/unset
-          // attribs on ALL sort columns
-        } else if (!value.IsEmpty()) {
-          child->UnsetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
-                           true);
-          child->UnsetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection,
-                           true);
-        }
+    if (child->IsXULElement(nsGkAtoms::treecols)) {
+      SetSortColumnHints(child, sortResource, sortDirection);
+    } else if (child->IsXULElement(nsGkAtoms::treecol)) {
+      nsAutoString value;
+      child->GetAttr(kNameSpaceID_None, nsGkAtoms::sort, value);
+      // also check the resource attribute for older code
+      if (value.IsEmpty())
+        child->GetAttr(kNameSpaceID_None, nsGkAtoms::resource, value);
+      if (value == sortResource) {
+        child->SetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
+                       NS_LITERAL_STRING("true"), true);
+        child->SetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection,
+                       sortDirection, true);
+        // Note: don't break out of loop; want to set/unset
+        // attribs on ALL sort columns
+      } else if (!value.IsEmpty()) {
+        child->UnsetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
+                         true);
+        child->UnsetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection,
+                         true);
       }
     }
   }
@@ -177,7 +173,7 @@ XULSortServiceImpl::GetTemplateItemsToSort(nsIContent* aContainer,
       cinfo->content = child;
       cinfo->result = result;
     }
-    else if (aContainer->Tag() != nsGkAtoms::_template) {
+    else if (!aContainer->IsXULElement(nsGkAtoms::_template)) {
       rv = GetTemplateItemsToSort(child, aBuilder, aSortState, aSortItems);
       NS_ENSURE_SUCCESS(rv, rv);
     }
diff --git a/dom/xul/templates/nsXULTemplateBuilder.cpp b/dom/xul/templates/nsXULTemplateBuilder.cpp
index 3287719922..dfeab490fe 100644
--- a/dom/xul/templates/nsXULTemplateBuilder.cpp
+++ b/dom/xul/templates/nsXULTemplateBuilder.cpp
@@ -773,7 +773,8 @@ nsXULTemplateBuilder::UpdateResultInContainer(nsIXULTemplateResult* aOldResult,
     if (aNewResult) {
         // only allow a result to be inserted into containers with a matching tag
         nsIAtom* tag = aQuerySet->GetTag();
-        if (aInsertionPoint && tag && tag != aInsertionPoint->Tag())
+        if (aInsertionPoint && tag &&
+            tag != aInsertionPoint->NodeInfo()->NameAtom())
             return NS_OK;
 
         int32_t findpriority = aQuerySet->Priority();
@@ -1243,7 +1244,7 @@ nsXULTemplateBuilder::LoadDataSources(nsIDocument* aDocument,
     if (xuldoc)
         xuldoc->SetTemplateBuilderFor(mRoot, this);
 
-    if (!mRoot->IsXUL()) {
+    if (!mRoot->IsXULElement()) {
         // Hmm. This must be an HTML element. Try to set it as a
         // JS property "by hand".
         InitHTMLTemplateRoot();
@@ -1438,7 +1439,8 @@ nsXULTemplateBuilder::DetermineMatchedRule(nsIContent *aContainer,
         // If a tag was specified, it must match the tag of the container
         // where content is being inserted.
         nsIAtom* tag = rule->GetTag();
-        if ((!aContainer || !tag || tag == aContainer->Tag()) &&
+        if ((!aContainer || !tag ||
+             tag == aContainer->NodeInfo()->NameAtom()) &&
             rule->CheckMatch(aResult)) {
             *aMatchedRule = rule;
             *aRuleIndex = r;
diff --git a/dom/xul/templates/nsXULTemplateQueryProcessorRDF.cpp b/dom/xul/templates/nsXULTemplateQueryProcessorRDF.cpp
index 5381631b85..b4a054cd9f 100644
--- a/dom/xul/templates/nsXULTemplateQueryProcessorRDF.cpp
+++ b/dom/xul/templates/nsXULTemplateQueryProcessorRDF.cpp
@@ -1220,7 +1220,7 @@ nsXULTemplateQueryProcessorRDF::CompileExtendedQuery(nsRDFQuery* aQuery,
          condition = condition->GetNextSibling()) {
 
         // the <content> condition should always be the first child
-        if (condition->Tag() == nsGkAtoms::content) {
+        if (condition->IsXULElement(nsGkAtoms::content)) {
             if (condition != aConditions->GetFirstChild()) {
                 nsXULContentUtils::LogTemplateError(ERROR_TEMPLATE_CONTENT_NOT_FIRST);
                 continue;
@@ -1245,8 +1245,8 @@ nsXULTemplateQueryProcessorRDF::CompileExtendedQuery(nsRDFQuery* aQuery,
         }
 
         TestNode* testnode = nullptr;
-        nsresult rv = CompileQueryChild(condition->Tag(), aQuery, condition,
-                                        prevnode, &testnode);
+        nsresult rv = CompileQueryChild(condition->NodeInfo()->NameAtom(),
+                                        aQuery, condition, prevnode, &testnode);
         if (NS_FAILED(rv))
             return rv;
 
diff --git a/editor/composer/nsComposeTxtSrvFilter.cpp b/editor/composer/nsComposeTxtSrvFilter.cpp
index aee839d864..418e0ccaab 100644
--- a/editor/composer/nsComposeTxtSrvFilter.cpp
+++ b/editor/composer/nsComposeTxtSrvFilter.cpp
@@ -28,13 +28,12 @@ nsComposeTxtSrvFilter::Skip(nsIDOMNode* aNode, bool *_retval)
   // their type is "cite"
   nsCOMPtr<nsIContent> content(do_QueryInterface(aNode));
   if (content) {
-    nsIAtom *tag = content->Tag();
-    if (tag == nsGkAtoms::blockquote) {
+    if (content->IsHTMLElement(nsGkAtoms::blockquote)) {
       if (mIsForMail) {
         *_retval = content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                                         nsGkAtoms::cite, eIgnoreCase);
       }
-    } else if (tag == nsGkAtoms::span) {
+    } else if (content->IsHTMLElement(nsGkAtoms::span)) {
       if (mIsForMail) {
         *_retval = content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::mozquote,
                                         nsGkAtoms::_true, eIgnoreCase);
@@ -43,12 +42,12 @@ nsComposeTxtSrvFilter::Skip(nsIDOMNode* aNode, bool *_retval)
                                           nsGkAtoms::mozsignature, eCaseMatters);
         }
       }
-    } else if (tag == nsGkAtoms::script ||
-               tag == nsGkAtoms::textarea ||
-               tag == nsGkAtoms::select ||
-               tag == nsGkAtoms::map) {
+    } else if (content->IsAnyOfHTMLElements(nsGkAtoms::script,
+                                            nsGkAtoms::textarea,
+                                            nsGkAtoms::select,
+                                            nsGkAtoms::map)) {
       *_retval = true;
-    } else if (tag == nsGkAtoms::table) {
+    } else if (content->IsHTMLElement(nsGkAtoms::table)) {
       if (mIsForMail) {
         *_retval =
           content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::_class,
diff --git a/editor/libeditor/nsEditor.cpp b/editor/libeditor/nsEditor.cpp
index a950e16a2d..ddc0930f5a 100644
--- a/editor/libeditor/nsEditor.cpp
+++ b/editor/libeditor/nsEditor.cpp
@@ -3140,7 +3140,7 @@ nsEditor::CanContain(nsINode& aParent, nsIContent& aChild)
   switch (aParent.NodeType()) {
   case nsIDOMNode::ELEMENT_NODE:
   case nsIDOMNode::DOCUMENT_FRAGMENT_NODE:
-    return TagCanContain(*aParent.Tag(), aChild);
+    return TagCanContain(*aParent.NodeInfo()->NameAtom(), aChild);
   }
   return false;
 }
@@ -3151,7 +3151,7 @@ nsEditor::CanContainTag(nsINode& aParent, nsIAtom& aChildTag)
   switch (aParent.NodeType()) {
   case nsIDOMNode::ELEMENT_NODE:
   case nsIDOMNode::DOCUMENT_FRAGMENT_NODE:
-    return TagCanContainTag(*aParent.Tag(), aChildTag);
+    return TagCanContainTag(*aParent.NodeInfo()->NameAtom(), aChildTag);
   }
   return false;
 }
@@ -3163,7 +3163,7 @@ nsEditor::TagCanContain(nsIAtom& aParentTag, nsIContent& aChild)
   case nsIDOMNode::TEXT_NODE:
   case nsIDOMNode::ELEMENT_NODE:
   case nsIDOMNode::DOCUMENT_FRAGMENT_NODE:
-    return TagCanContainTag(aParentTag, *aChild.Tag());
+    return TagCanContainTag(aParentTag, *aChild.NodeInfo()->NameAtom());
   }
   return false;
 }
@@ -3413,12 +3413,12 @@ nsEditor::GetTag(nsIDOMNode *aNode)
 
   if (!content) 
   {
-    NS_ASSERTION(aNode, "null node passed to nsEditor::Tag()");
+    NS_ASSERTION(aNode, "null node passed to nsEditor::GetTag()");
 
     return nullptr;
   }
   
-  return content->Tag();
+  return content->NodeInfo()->NameAtom();
 }
 
 
@@ -3430,7 +3430,7 @@ nsEditor::GetTagString(nsIDOMNode *aNode, nsAString& outString)
 {
   if (!aNode) 
   {
-    NS_NOTREACHED("null node passed to nsEditor::GetTag()");
+    NS_NOTREACHED("null node passed to nsEditor::GetTagString()");
     return NS_ERROR_NULL_POINTER;
   }
   
@@ -3471,7 +3471,7 @@ nsEditor::AreNodesSameType(nsIContent* aNode1, nsIContent* aNode2)
 {
   MOZ_ASSERT(aNode1);
   MOZ_ASSERT(aNode2);
-  return aNode1->Tag() == aNode2->Tag();
+  return aNode1->NodeInfo()->NameAtom() == aNode2->NodeInfo()->NameAtom();
 }
 
 
diff --git a/editor/libeditor/nsEditorEventListener.cpp b/editor/libeditor/nsEditorEventListener.cpp
index c7ec616af9..5ef1b14758 100644
--- a/editor/libeditor/nsEditorEventListener.cpp
+++ b/editor/libeditor/nsEditorEventListener.cpp
@@ -1144,7 +1144,7 @@ nsEditorEventListener::IsFileControlTextBox()
     return false;
   }
   nsIContent* parent = root->FindFirstNonChromeOnlyAccessContent();
-  if (!parent || !parent->IsHTML(nsGkAtoms::input)) {
+  if (!parent || !parent->IsHTMLElement(nsGkAtoms::input)) {
     return false;
   }
   nsCOMPtr<nsIFormControl> formControl = do_QueryInterface(parent);
diff --git a/editor/libeditor/nsHTMLAbsPosition.cpp b/editor/libeditor/nsHTMLAbsPosition.cpp
index 467d8cb662..7e520e8ab6 100644
--- a/editor/libeditor/nsHTMLAbsPosition.cpp
+++ b/editor/libeditor/nsHTMLAbsPosition.cpp
@@ -553,7 +553,8 @@ nsHTMLEditor::AbsolutelyPositionElement(nsIDOMElement* aElement,
     }
 
     nsCOMPtr<dom::Element> element = do_QueryInterface(aElement);
-    if (element && element->IsHTML(nsGkAtoms::div) && !HasStyleOrIdOrClass(element)) {
+    if (element && element->IsHTMLElement(nsGkAtoms::div) &&
+        !HasStyleOrIdOrClass(element)) {
       nsRefPtr<nsHTMLEditRules> htmlRules = static_cast<nsHTMLEditRules*>(mRules.get());
       NS_ENSURE_TRUE(htmlRules, NS_ERROR_FAILURE);
       nsresult res = htmlRules->MakeSureElemStartsOrEndsOnCR(aElement);
diff --git a/editor/libeditor/nsHTMLCSSUtils.cpp b/editor/libeditor/nsHTMLCSSUtils.cpp
index 349a65cc81..e0d2cd27a5 100644
--- a/editor/libeditor/nsHTMLCSSUtils.cpp
+++ b/editor/libeditor/nsHTMLCSSUtils.cpp
@@ -326,9 +326,6 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
     NS_ENSURE_TRUE(content, false);
   }
 
-  nsIAtom *tagName = content->Tag();
-  // brade: shouldn't some of the above go below the next block?
-
   // html inline styles B I TT U STRIKE and COLOR/FACE on FONT
   if (nsGkAtoms::b == aProperty ||
       nsGkAtoms::i == aProperty ||
@@ -343,41 +340,43 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
 
   // ALIGN attribute on elements supporting it
   if (aAttribute && (aAttribute->EqualsLiteral("align")) &&
-      (nsGkAtoms::div == tagName ||
-       nsGkAtoms::p   == tagName ||
-       nsGkAtoms::h1  == tagName ||
-       nsGkAtoms::h2  == tagName ||
-       nsGkAtoms::h3  == tagName ||
-       nsGkAtoms::h4  == tagName ||
-       nsGkAtoms::h5  == tagName ||
-       nsGkAtoms::h6  == tagName ||
-       nsGkAtoms::td  == tagName ||
-       nsGkAtoms::th  == tagName ||
-       nsGkAtoms::table  == tagName ||
-       nsGkAtoms::hr  == tagName ||
-       // brade: for the above, why not use nsHTMLEditUtils::SupportsAlignAttr
-       // brade: but it also checks for tbody, tfoot, thead
-       // Let's add the following elements here even if ALIGN has not
-       // the same meaning for them
-       nsGkAtoms::legend  == tagName ||
-       nsGkAtoms::caption == tagName)) {
+      content->IsAnyOfHTMLElements(nsGkAtoms::div,
+                                   nsGkAtoms::p ,
+                                   nsGkAtoms::h1,
+                                   nsGkAtoms::h2,
+                                   nsGkAtoms::h3,
+                                   nsGkAtoms::h4,
+                                   nsGkAtoms::h5,
+                                   nsGkAtoms::h6,
+                                   nsGkAtoms::td,
+                                   nsGkAtoms::th,
+                                   nsGkAtoms::table,
+                                   nsGkAtoms::hr,
+				   // brade: for the above, why not use
+				   // nsHTMLEditUtils::SupportsAlignAttr
+				   // brade: but it also checks for tbody,
+				   // tfoot, thead Let's add the following
+				   // elements here even if ALIGN has not the
+				   // same meaning for them
+                                   nsGkAtoms::legend,
+                                   nsGkAtoms::caption)) {
     return true;
   }
 
   if (aAttribute && (aAttribute->EqualsLiteral("valign")) &&
-      (nsGkAtoms::col == tagName ||
-       nsGkAtoms::colgroup   == tagName ||
-       nsGkAtoms::tbody  == tagName ||
-       nsGkAtoms::td  == tagName ||
-       nsGkAtoms::th  == tagName ||
-       nsGkAtoms::tfoot  == tagName ||
-       nsGkAtoms::thead  == tagName ||
-       nsGkAtoms::tr  == tagName)) {
+      content->IsAnyOfHTMLElements(nsGkAtoms::col,
+                                   nsGkAtoms::colgroup,
+                                   nsGkAtoms::tbody,
+                                   nsGkAtoms::td,
+                                   nsGkAtoms::th,
+                                   nsGkAtoms::tfoot,
+                                   nsGkAtoms::thead,
+                                   nsGkAtoms::tr)) {
     return true;
   }
 
   // attributes TEXT, BACKGROUND and BGCOLOR on BODY
-  if (aAttribute && nsGkAtoms::body == tagName &&
+  if (aAttribute && content->IsHTMLElement(nsGkAtoms::body) &&
       (aAttribute->EqualsLiteral("text")
        || aAttribute->EqualsLiteral("background")
        || aAttribute->EqualsLiteral("bgcolor"))) {
@@ -390,7 +389,8 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
   }
 
   // attributes HEIGHT, WIDTH and NOWRAP on TD and TH
-  if (aAttribute && (nsGkAtoms::td == tagName || nsGkAtoms::th == tagName) &&
+  if (aAttribute &&
+      content->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th) &&
       (aAttribute->EqualsLiteral("height")
        || aAttribute->EqualsLiteral("width")
        || aAttribute->EqualsLiteral("nowrap"))) {
@@ -398,14 +398,14 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
   }
 
   // attributes HEIGHT and WIDTH on TABLE
-  if (aAttribute && nsGkAtoms::table == tagName &&
+  if (aAttribute && content->IsHTMLElement(nsGkAtoms::table) &&
       (aAttribute->EqualsLiteral("height")
        || aAttribute->EqualsLiteral("width"))) {
     return true;
   }
 
   // attributes SIZE and WIDTH on HR
-  if (aAttribute && nsGkAtoms::hr == tagName &&
+  if (aAttribute && content->IsHTMLElement(nsGkAtoms::hr) &&
       (aAttribute->EqualsLiteral("size")
        || aAttribute->EqualsLiteral("width"))) {
     return true;
@@ -413,12 +413,13 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
 
   // attribute TYPE on OL UL LI
   if (aAttribute &&
-      (nsGkAtoms::ol == tagName || nsGkAtoms::ul == tagName ||
-       nsGkAtoms::li == tagName) && aAttribute->EqualsLiteral("type")) {
+      content->IsAnyOfHTMLElements(nsGkAtoms::ol, nsGkAtoms::ul,
+                                   nsGkAtoms::li) &&
+      aAttribute->EqualsLiteral("type")) {
     return true;
   }
 
-  if (aAttribute && nsGkAtoms::img == tagName &&
+  if (aAttribute && content->IsHTMLElement(nsGkAtoms::img) &&
       (aAttribute->EqualsLiteral("border")
        || aAttribute->EqualsLiteral("width")
        || aAttribute->EqualsLiteral("height"))) {
@@ -428,15 +429,15 @@ nsHTMLCSSUtils::IsCSSEditableProperty(nsIContent* aNode,
   // other elements that we can align using CSS even if they
   // can't carry the html ALIGN attribute
   if (aAttribute && aAttribute->EqualsLiteral("align") &&
-      (nsGkAtoms::ul == tagName ||
-       nsGkAtoms::ol == tagName ||
-       nsGkAtoms::dl == tagName ||
-       nsGkAtoms::li == tagName ||
-       nsGkAtoms::dd == tagName ||
-       nsGkAtoms::dt == tagName ||
-       nsGkAtoms::address == tagName ||
-       nsGkAtoms::pre == tagName ||
-       nsGkAtoms::ul == tagName)) {
+      content->IsAnyOfHTMLElements(nsGkAtoms::ul,
+                                   nsGkAtoms::ol,
+                                   nsGkAtoms::dl,
+                                   nsGkAtoms::li,
+                                   nsGkAtoms::dd,
+                                   nsGkAtoms::dt,
+                                   nsGkAtoms::address,
+                                   nsGkAtoms::pre,
+                                   nsGkAtoms::ul)) {
     return true;
   }
 
@@ -588,7 +589,7 @@ nsHTMLCSSUtils::RemoveCSSInlineStyle(nsIDOMNode *aNode, nsIAtom *aProperty, cons
   nsresult res = RemoveCSSProperty(*element, *aProperty, aPropertyValue);
   NS_ENSURE_SUCCESS(res, res);
 
-  if (!element->IsHTML(nsGkAtoms::span) ||
+  if (!element->IsHTMLElement(nsGkAtoms::span) ||
       nsHTMLEditor::HasAttributes(element)) {
     return NS_OK;
   }
@@ -810,7 +811,6 @@ nsHTMLCSSUtils::GenerateCSSDeclarationsFromHTMLStyle(dom::Element* aElement,
                                                      bool aGetOrRemoveRequest)
 {
   MOZ_ASSERT(aElement);
-  nsIAtom* tagName = aElement->Tag();
   const nsHTMLCSSUtils::CSSEquivTable* equivTable = nullptr;
 
   if (nsGkAtoms::b == aHTMLProperty) {
@@ -839,12 +839,12 @@ nsHTMLCSSUtils::GenerateCSSDeclarationsFromHTMLStyle(dom::Element* aElement,
     } else if (aAttribute->EqualsLiteral("border")) {
       equivTable = borderEquivTable;
     } else if (aAttribute->EqualsLiteral("align")) {
-      if (nsGkAtoms::table  == tagName) {
+      if (aElement->IsHTMLElement(nsGkAtoms::table)) {
         equivTable = tableAlignEquivTable;
-      } else if (nsGkAtoms::hr  == tagName) {
+      } else if (aElement->IsHTMLElement(nsGkAtoms::hr)) {
         equivTable = hrAlignEquivTable;
-      } else if (nsGkAtoms::legend  == tagName ||
-                 nsGkAtoms::caption == tagName) {
+      } else if (aElement->IsAnyOfHTMLElements(nsGkAtoms::legend,
+                                               nsGkAtoms::caption)) {
         equivTable = captionAlignEquivTable;
       } else {
         equivTable = textAlignEquivTable;
@@ -856,13 +856,13 @@ nsHTMLCSSUtils::GenerateCSSDeclarationsFromHTMLStyle(dom::Element* aElement,
     } else if (aAttribute->EqualsLiteral("width")) {
       equivTable = widthEquivTable;
     } else if (aAttribute->EqualsLiteral("height") ||
-               (nsGkAtoms::hr == tagName &&
+               (aElement->IsHTMLElement(nsGkAtoms::hr) &&
                 aAttribute->EqualsLiteral("size"))) {
       equivTable = heightEquivTable;
     } else if (aAttribute->EqualsLiteral("type") &&
-               (nsGkAtoms::ol == tagName ||
-                nsGkAtoms::ul == tagName ||
-                nsGkAtoms::li == tagName)) {
+               aElement->IsAnyOfHTMLElements(nsGkAtoms::ol,
+                                             nsGkAtoms::ul,
+                                             nsGkAtoms::li)) {
       equivTable = listStyleTypeEquivTable;
     }
   }
diff --git a/editor/libeditor/nsHTMLDataTransfer.cpp b/editor/libeditor/nsHTMLDataTransfer.cpp
index e7baf77d35..fa301559d2 100644
--- a/editor/libeditor/nsHTMLDataTransfer.cpp
+++ b/editor/libeditor/nsHTMLDataTransfer.cpp
@@ -2070,8 +2070,8 @@ nsresult nsHTMLEditor::CreateDOMFragmentFromPaste(const nsAString &aInputString,
   // create fragment for pasted html
   nsIAtom* contextAtom;
   if (contextLeafAsContent) {
-    contextAtom = contextLeafAsContent->Tag();
-    if (contextAtom == nsGkAtoms::html) {
+    contextAtom = contextLeafAsContent->NodeInfo()->NameAtom();
+    if (contextLeafAsContent->IsHTMLElement(nsGkAtoms::html)) {
       contextAtom = nsGkAtoms::body;
     }
   } else {
diff --git a/editor/libeditor/nsHTMLEditRules.cpp b/editor/libeditor/nsHTMLEditRules.cpp
index 5e09602698..2411733ab2 100644
--- a/editor/libeditor/nsHTMLEditRules.cpp
+++ b/editor/libeditor/nsHTMLEditRules.cpp
@@ -703,21 +703,21 @@ nsHTMLEditRules::GetListState(bool *aMixed, bool *aOL, bool *aUL, bool *aDL)
 
     if (!curElement) {
       bNonList = true;
-    } else if (curElement->IsHTML(nsGkAtoms::ul)) {
+    } else if (curElement->IsHTMLElement(nsGkAtoms::ul)) {
       *aUL = true;
-    } else if (curElement->IsHTML(nsGkAtoms::ol)) {
+    } else if (curElement->IsHTMLElement(nsGkAtoms::ol)) {
       *aOL = true;
-    } else if (curElement->IsHTML(nsGkAtoms::li)) {
+    } else if (curElement->IsHTMLElement(nsGkAtoms::li)) {
       if (dom::Element* parent = curElement->GetParentElement()) {
-        if (parent->IsHTML(nsGkAtoms::ul)) {
+        if (parent->IsHTMLElement(nsGkAtoms::ul)) {
           *aUL = true;
-        } else if (parent->IsHTML(nsGkAtoms::ol)) {
+        } else if (parent->IsHTMLElement(nsGkAtoms::ol)) {
           *aOL = true;
         }
       }
-    } else if (curElement->IsHTML(nsGkAtoms::dl) ||
-               curElement->IsHTML(nsGkAtoms::dt) ||
-               curElement->IsHTML(nsGkAtoms::dd)) {
+    } else if (curElement->IsAnyOfHTMLElements(nsGkAtoms::dl,
+                                               nsGkAtoms::dt,
+                                               nsGkAtoms::dd)) {
       *aDL = true;
     } else {
       bNonList = true;
@@ -753,15 +753,15 @@ nsHTMLEditRules::GetListItemState(bool *aMixed, bool *aLI, bool *aDT, bool *aDD)
     nsCOMPtr<dom::Element> element = do_QueryInterface(curNode);
     if (!element) {
       bNonList = true;
-    } else if (element->IsHTML(nsGkAtoms::ul) ||
-               element->IsHTML(nsGkAtoms::ol) ||
-               element->IsHTML(nsGkAtoms::li)) {
+    } else if (element->IsAnyOfHTMLElements(nsGkAtoms::ul,
+                                            nsGkAtoms::ol,
+                                            nsGkAtoms::li)) {
       *aLI = true;
-    } else if (element->IsHTML(nsGkAtoms::dt)) {
+    } else if (element->IsHTMLElement(nsGkAtoms::dt)) {
       *aDT = true;
-    } else if (element->IsHTML(nsGkAtoms::dd)) {
+    } else if (element->IsHTMLElement(nsGkAtoms::dd)) {
       *aDD = true;
-    } else if (element->IsHTML(nsGkAtoms::dl)) {
+    } else if (element->IsHTMLElement(nsGkAtoms::dl)) {
       // need to look inside dl and see which types of items it has
       bool bDT, bDD;
       GetDefinitionListItemTypes(element, &bDT, &bDD);
@@ -826,7 +826,7 @@ nsHTMLEditRules::GetAlignment(bool *aMixed, nsIHTMLEditor::EAlignment *aAlign)
   {
     // if we are in a text node, then that is the node of interest
     nodeToExamine = GetAsDOMNode(parent);
-  } else if (parent->Tag() == nsGkAtoms::html && offset == rootOffset) {
+  } else if (parent->IsHTMLElement(nsGkAtoms::html) && offset == rootOffset) {
     // if we have selected the body, let's look at the first editable node
     NS_ENSURE_STATE(mHTMLEditor);
     nodeToExamine =
@@ -2055,9 +2055,9 @@ nsHTMLEditRules::WillDeleteSelection(Selection* aSelection,
     }
     
     if (wsType == WSType::special || wsType == WSType::br ||
-        visNode->Tag() == nsGkAtoms::hr) {
+        visNode->IsHTMLElement(nsGkAtoms::hr)) {
       // Short circuit for invisible breaks.  delete them and recurse.
-      if (visNode->Tag() == nsGkAtoms::br &&
+      if (visNode->IsHTMLElement(nsGkAtoms::br) &&
           (!mHTMLEditor || !mHTMLEditor->IsVisBreak(visNode))) {
         NS_ENSURE_STATE(mHTMLEditor);
         res = mHTMLEditor->DeleteNode(visNode);
@@ -2067,7 +2067,8 @@ nsHTMLEditRules::WillDeleteSelection(Selection* aSelection,
       }
 
       // Special handling for backspace when positioned after <hr>
-      if (aAction == nsIEditor::ePrevious && visNode->Tag() == nsGkAtoms::hr) {
+      if (aAction == nsIEditor::ePrevious &&
+          visNode->IsHTMLElement(nsGkAtoms::hr)) {
         // Only if the caret is positioned at the end-of-hr-line position, we
         // want to delete the <hr>.
         //
@@ -2210,7 +2211,7 @@ nsHTMLEditRules::WillDeleteSelection(Selection* aSelection,
         rightNode = leafNode;
       }
 
-      if (otherNode->Tag() == nsGkAtoms::br) {
+      if (otherNode->IsHTMLElement(nsGkAtoms::br)) {
         NS_ENSURE_STATE(mHTMLEditor);
         res = mHTMLEditor->DeleteNode(otherNode);
         NS_ENSURE_SUCCESS(res, res);
@@ -2391,7 +2392,7 @@ nsHTMLEditRules::WillDeleteSelection(Selection* aSelection,
         if (leftBlockParent == rightBlockParent &&
             mHTMLEditor->NodesSameType(GetAsDOMNode(leftParent),
                                        GetAsDOMNode(rightParent))) {
-          if (leftParent->Tag() == nsGkAtoms::p) {
+          if (leftParent->IsHTMLElement(nsGkAtoms::p)) {
             // First delete the selection
             NS_ENSURE_STATE(mHTMLEditor);
             res = mHTMLEditor->DeleteSelectionImpl(aAction, aStripWrappers);
@@ -2459,7 +2460,7 @@ nsHTMLEditRules::WillDeleteSelection(Selection* aSelection,
                 mHTMLEditor->IsVisTextNode(content, &join, true);
               } else {
                 NS_ENSURE_STATE(mHTMLEditor);
-                join = content->IsHTML(nsGkAtoms::br) &&
+                join = content->IsHTMLElement(nsGkAtoms::br) &&
                        !mHTMLEditor->IsVisBreak(somenode);
               }
             }
@@ -2577,7 +2578,8 @@ nsHTMLEditRules::GetGoodSelPointForNode(nsINode& aNode,
   ret.node = aNode.GetParentNode();
   ret.offset = ret.node ? ret.node->IndexOf(&aNode) : -1;
   NS_ENSURE_TRUE(mHTMLEditor, ::DOMPoint());
-  if ((aNode.Tag() != nsGkAtoms::br || mHTMLEditor->IsVisBreak(&aNode)) &&
+  if ((!aNode.IsHTMLElement(nsGkAtoms::br) ||
+       mHTMLEditor->IsVisBreak(&aNode)) &&
       aAction == nsIEditor::ePrevious) {
     ret.offset++;
   }
@@ -3221,7 +3223,7 @@ nsHTMLEditRules::WillMakeList(Selection* aSelection,
 
     if (nsHTMLEditUtils::IsListItem(curNode)) {
       NS_ENSURE_STATE(mHTMLEditor);
-      if (curParent->Tag() != listType) {
+      if (!curParent->IsHTMLElement(listType)) {
         // list item is in wrong type of list. if we don't have a curList,
         // split the old list and make a new list of correct type.
         if (!curList || nsEditorUtils::IsDescendantOf(curNode, curList)) {
@@ -3242,7 +3244,7 @@ nsHTMLEditRules::WillMakeList(Selection* aSelection,
         NS_ENSURE_SUCCESS(res, res);
         // convert list item type if needed
         NS_ENSURE_STATE(mHTMLEditor);
-        if (curNode->Tag() != itemType) {
+        if (!curNode->IsHTMLElement(itemType)) {
           NS_ENSURE_STATE(mHTMLEditor);
           newBlock = dont_AddRef(GetAsDOMNode(
             mHTMLEditor->ReplaceContainer(curNode->AsElement(),
@@ -3261,7 +3263,7 @@ nsHTMLEditRules::WillMakeList(Selection* aSelection,
           NS_ENSURE_SUCCESS(res, res);
         }
         NS_ENSURE_STATE(mHTMLEditor);
-        if (curNode->Tag() != itemType) {
+        if (!curNode->IsHTMLElement(itemType)) {
           NS_ENSURE_STATE(mHTMLEditor);
           newBlock = dont_AddRef(GetAsDOMNode(
             mHTMLEditor->ReplaceContainer(curNode->AsElement(),
@@ -3284,7 +3286,7 @@ nsHTMLEditRules::WillMakeList(Selection* aSelection,
 
     // if we hit a div clear our prevListItem, insert divs contents
     // into our node array, and remove the div
-    if (curNode->Tag() == nsGkAtoms::div) {
+    if (curNode->IsHTMLElement(nsGkAtoms::div)) {
       prevListItem = nullptr;
       int32_t j = i + 1;
       res = GetInnerContent(curNode->AsDOMNode(), arrayOfNodes, &j);
@@ -3320,7 +3322,7 @@ nsHTMLEditRules::WillMakeList(Selection* aSelection,
         NS_ENSURE_SUCCESS(res, res);
       } else {
         // don't wrap li around a paragraph.  instead replace paragraph with li
-        if (curNode->Tag() == nsGkAtoms::p) {
+        if (curNode->IsHTMLElement(nsGkAtoms::p)) {
           NS_ENSURE_STATE(mHTMLEditor);
           listItem = mHTMLEditor->ReplaceContainer(curNode->AsElement(),
                                                    itemType);
@@ -3747,7 +3749,8 @@ nsHTMLEditRules::WillCSSIndent(Selection* aSelection,
       sibling = mHTMLEditor->GetNextHTMLSibling(curNode);
       if (sibling && nsHTMLEditUtils::IsList(sibling))
       {
-        if (curParent->Tag() == sibling->Tag()) {
+        if (curParent->NodeInfo()->NameAtom() == sibling->NodeInfo()->NameAtom() &&
+            curParent->NodeInfo()->NamespaceID() == sibling->NodeInfo()->NamespaceID()) {
           NS_ENSURE_STATE(mHTMLEditor);
           res = mHTMLEditor->MoveNode(curNode, sibling, 0);
           NS_ENSURE_SUCCESS(res, res);
@@ -3761,7 +3764,8 @@ nsHTMLEditRules::WillCSSIndent(Selection* aSelection,
       sibling = mHTMLEditor->GetPriorHTMLSibling(curNode);
       if (sibling && nsHTMLEditUtils::IsList(sibling))
       {
-        if (curParent->Tag() == sibling->Tag()) {
+        if (curParent->NodeInfo()->NameAtom() == sibling->NodeInfo()->NameAtom() &&
+            curParent->NodeInfo()->NamespaceID() == sibling->NodeInfo()->NamespaceID()) {
           NS_ENSURE_STATE(mHTMLEditor);
           res = mHTMLEditor->MoveNode(curNode, sibling, -1);
           NS_ENSURE_SUCCESS(res, res);
@@ -3779,10 +3783,12 @@ nsHTMLEditRules::WillCSSIndent(Selection* aSelection,
 
       if (!curList || (sibling && sibling != curList)) {
         // create a new nested list of correct type
-        res = SplitAsNeeded(*curParent->Tag(), curParent, offset);
+        res = SplitAsNeeded(*curParent->NodeInfo()->NameAtom(), curParent,
+                            offset);
         NS_ENSURE_SUCCESS(res, res);
         NS_ENSURE_STATE(mHTMLEditor);
-        curList = mHTMLEditor->CreateNode(curParent->Tag(), curParent, offset);
+        curList = mHTMLEditor->CreateNode(curParent->NodeInfo()->NameAtom(),
+                                          curParent, offset);
         NS_ENSURE_STATE(curList);
         // curList is now the correct thing to put curNode in
         // remember our new block for postprocessing
@@ -3929,7 +3935,8 @@ nsHTMLEditRules::WillHTMLIndent(Selection* aSelection,
       NS_ENSURE_STATE(mHTMLEditor);
       sibling = mHTMLEditor->GetNextHTMLSibling(curNode);
       if (sibling && nsHTMLEditUtils::IsList(sibling) &&
-          curParent->Tag() == sibling->Tag()) {
+          curParent->NodeInfo()->NameAtom() == sibling->NodeInfo()->NameAtom() &&
+          curParent->NodeInfo()->NamespaceID() == sibling->NodeInfo()->NamespaceID()) {
         NS_ENSURE_STATE(mHTMLEditor);
         res = mHTMLEditor->MoveNode(curNode, sibling, 0);
         NS_ENSURE_SUCCESS(res, res);
@@ -3942,7 +3949,8 @@ nsHTMLEditRules::WillHTMLIndent(Selection* aSelection,
       NS_ENSURE_STATE(mHTMLEditor);
       sibling = mHTMLEditor->GetPriorHTMLSibling(curNode);
       if (sibling && nsHTMLEditUtils::IsList(sibling) &&
-          curParent->Tag() == sibling->Tag()) {
+          curParent->NodeInfo()->NameAtom() == sibling->NodeInfo()->NameAtom() &&
+          curParent->NodeInfo()->NamespaceID() == sibling->NodeInfo()->NamespaceID()) {
         NS_ENSURE_STATE(mHTMLEditor);
         res = mHTMLEditor->MoveNode(curNode, sibling, -1);
         NS_ENSURE_SUCCESS(res, res);
@@ -3961,10 +3969,12 @@ nsHTMLEditRules::WillHTMLIndent(Selection* aSelection,
       if (!curList || (sibling && sibling != curList) )
       {
         // create a new nested list of correct type
-        res = SplitAsNeeded(*curParent->Tag(), curParent, offset);
+        res = SplitAsNeeded(*curParent->NodeInfo()->NameAtom(), curParent,
+                            offset);
         NS_ENSURE_SUCCESS(res, res);
         NS_ENSURE_STATE(mHTMLEditor);
-        curList = mHTMLEditor->CreateNode(curParent->Tag(), curParent, offset);
+        curList = mHTMLEditor->CreateNode(curParent->NodeInfo()->NameAtom(),
+                                          curParent, offset);
         NS_ENSURE_STATE(curList);
         // curList is now the correct thing to put curNode in
         // remember our new block for postprocessing
@@ -4006,11 +4016,12 @@ nsHTMLEditRules::WillHTMLIndent(Selection* aSelection,
         if (!curList || (sibling && sibling != curList) )
         {
           // create a new nested list of correct type
-          res = SplitAsNeeded(*curParent->Tag(), curParent, offset);
+          res = SplitAsNeeded(*curParent->NodeInfo()->NameAtom(), curParent,
+                              offset);
           NS_ENSURE_SUCCESS(res, res);
           NS_ENSURE_STATE(mHTMLEditor);
-          curList = mHTMLEditor->CreateNode(curParent->Tag(), curParent,
-                                            offset);
+          curList = mHTMLEditor->CreateNode(curParent->NodeInfo()->NameAtom(),
+                                            curParent, offset);
           NS_ENSURE_STATE(curList);
         }
         NS_ENSURE_STATE(mHTMLEditor);
@@ -4485,11 +4496,12 @@ nsHTMLEditRules::ConvertListType(Element* aList,
   {
     if (child->IsElement()) {
       dom::Element* element = child->AsElement();
-      if (nsHTMLEditUtils::IsListItem(element) && !element->IsHTML(aItemType)) {
+      if (nsHTMLEditUtils::IsListItem(element) &&
+          !element->IsHTMLElement(aItemType)) {
         child = mHTMLEditor->ReplaceContainer(element, aItemType);
         NS_ENSURE_STATE(child);
       } else if (nsHTMLEditUtils::IsList(element) &&
-                 !element->IsHTML(aListType)) {
+                 !element->IsHTMLElement(aListType)) {
         nsCOMPtr<dom::Element> temp;
         nsresult rv = ConvertListType(child->AsElement(), getter_AddRefs(temp),
                                       aListType, aItemType);
@@ -4500,7 +4512,7 @@ nsHTMLEditRules::ConvertListType(Element* aList,
     child = child->GetNextSibling();
   }
 
-  if (aList->IsElement() && aList->AsElement()->IsHTML(aListType)) {
+  if (aList->IsHTMLElement(aListType)) {
     nsCOMPtr<dom::Element> list = aList->AsElement();
     list.forget(aOutList);
     return NS_OK;
@@ -4959,7 +4971,8 @@ nsHTMLEditRules::AlignBlockContents(nsIDOMNode *aNode, const nsAString *alignTyp
   if (!firstChild)
   {
     // this cell has no content, nothing to align
-  } else if (firstChild == lastChild && firstChild->Tag() == nsGkAtoms::div) {
+  } else if (firstChild == lastChild &&
+             firstChild->IsHTMLElement(nsGkAtoms::div)) {
     // the cell already has a div containing all of its content: just
     // act on this div.
     nsCOMPtr<nsIDOMElement> divElem = do_QueryInterface(firstChild);
@@ -5622,14 +5635,14 @@ nsHTMLEditRules::GetPromotedPoint(RulesEndpoint aWhere, nsIDOMNode* aNode,
     NS_ENSURE_TRUE(mHTMLEditor, /* void */);
     nsCOMPtr<nsIContent> nearNode =
       mHTMLEditor->GetPriorHTMLNode(node, offset, true);
-    while (!nearNode && node->Tag() != nsGkAtoms::body &&
+    while (!nearNode && !node->IsHTMLElement(nsGkAtoms::body) &&
            node->GetParentNode()) {
       // some cutoffs are here: we don't need to also include them in the
       // aWhere == kEnd case.  as long as they are in one or the other it will
       // work.  special case for outdent: don't keep looking up if we have
       // found a blockquote element to act on
       if (actionID == EditAction::outdent &&
-          node->Tag() == nsGkAtoms::blockquote) {
+          node->IsHTMLElement(nsGkAtoms::blockquote)) {
         break;
       }
 
@@ -5698,7 +5711,7 @@ nsHTMLEditRules::GetPromotedPoint(RulesEndpoint aWhere, nsIDOMNode* aNode,
   NS_ENSURE_TRUE(mHTMLEditor, /* void */);
   nsCOMPtr<nsIContent> nearNode =
     mHTMLEditor->GetNextHTMLNode(node, offset, true);
-  while (!nearNode && node->Tag() != nsGkAtoms::body &&
+  while (!nearNode && !node->IsHTMLElement(nsGkAtoms::body) &&
          node->GetParentNode()) {
     int32_t parentOffset = node->GetParentNode()->IndexOf(node);
     nsCOMPtr<nsINode> parent = node->GetParentNode();
@@ -6159,9 +6172,9 @@ nsHTMLEditRules::LookInsideDivBQandList(nsCOMArray<nsIDOMNode>& aNodeArray)
   NS_ENSURE_STATE(curNode);
 
   while (curNode->IsElement() &&
-         (curNode->AsElement()->IsHTML(nsGkAtoms::div) ||
+         (curNode->IsHTMLElement(nsGkAtoms::div) ||
           nsHTMLEditUtils::IsList(curNode) ||
-          curNode->AsElement()->IsHTML(nsGkAtoms::blockquote))) {
+          curNode->IsHTMLElement(nsGkAtoms::blockquote))) {
     // dive as long as there is only one child, and it is a list, div, blockquote
     NS_ENSURE_STATE(mHTMLEditor);
     uint32_t numChildren = mHTMLEditor->CountEditableChildren(curNode);
@@ -6177,9 +6190,9 @@ nsHTMLEditRules::LookInsideDivBQandList(nsCOMArray<nsIDOMNode>& aNodeArray)
     }
 
     dom::Element* element = tmp->AsElement();
-    if (!element->IsHTML(nsGkAtoms::div) &&
+    if (!element->IsHTMLElement(nsGkAtoms::div) &&
         !nsHTMLEditUtils::IsList(element) &&
-        !element->IsHTML(nsGkAtoms::blockquote)) {
+        !element->IsHTMLElement(nsGkAtoms::blockquote)) {
       break;
     }
 
@@ -6190,9 +6203,8 @@ nsHTMLEditRules::LookInsideDivBQandList(nsCOMArray<nsIDOMNode>& aNodeArray)
   // we've found innermost list/blockquote/div: 
   // replace the one node in the array with these nodes
   aNodeArray.RemoveObjectAt(0);
-  if (curNode->IsElement() &&
-      (curNode->AsElement()->IsHTML(nsGkAtoms::div) ||
-       curNode->AsElement()->IsHTML(nsGkAtoms::blockquote))) {
+  if (curNode->IsAnyOfHTMLElements(nsGkAtoms::div,
+                                   nsGkAtoms::blockquote)) {
     int32_t j = 0;
     return GetInnerContent(curNode->AsDOMNode(), aNodeArray, &j, false, false);
   }
@@ -6209,7 +6221,7 @@ void
 nsHTMLEditRules::GetDefinitionListItemTypes(dom::Element* aElement, bool* aDT, bool* aDD)
 {
   MOZ_ASSERT(aElement);
-  MOZ_ASSERT(aElement->IsHTML(nsGkAtoms::dl));
+  MOZ_ASSERT(aElement->IsHTMLElement(nsGkAtoms::dl));
   MOZ_ASSERT(aDT);
   MOZ_ASSERT(aDD);
 
@@ -6217,9 +6229,9 @@ nsHTMLEditRules::GetDefinitionListItemTypes(dom::Element* aElement, bool* aDT, b
   for (nsIContent* child = aElement->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(nsGkAtoms::dt)) {
+    if (child->IsHTMLElement(nsGkAtoms::dt)) {
       *aDT = true;
-    } else if (child->IsHTML(nsGkAtoms::dd)) {
+    } else if (child->IsHTMLElement(nsGkAtoms::dd)) {
       *aDD = true;
     }
   }
@@ -7124,14 +7136,14 @@ nsHTMLEditRules::RemoveBlockStyle(nsCOMArray<nsIDOMNode>& arrayOfNodes)
       res = mHTMLEditor->RemoveBlockContainer(curNode); 
       NS_ENSURE_SUCCESS(res, res);
     } else if (curElement &&
-               (curElement->IsHTML(nsGkAtoms::table)      ||
-                curElement->IsHTML(nsGkAtoms::tr)         ||
-                curElement->IsHTML(nsGkAtoms::tbody)      ||
-                curElement->IsHTML(nsGkAtoms::td)         ||
-                nsHTMLEditUtils::IsList(curElement)       ||
-                curElement->IsHTML(nsGkAtoms::li)         ||
-                curElement->IsHTML(nsGkAtoms::blockquote) ||
-                curElement->IsHTML(nsGkAtoms::div))) {
+               (curElement->IsAnyOfHTMLElements(nsGkAtoms::table,
+                                                nsGkAtoms::tr,
+                                                nsGkAtoms::tbody,
+                                                nsGkAtoms::td,
+                                                nsGkAtoms::li,
+                                                nsGkAtoms::blockquote,
+                                                nsGkAtoms::div) ||
+                nsHTMLEditUtils::IsList(curElement))) {
       // process any partial progress saved
       if (curBlock)
       {
@@ -7243,7 +7255,7 @@ nsHTMLEditRules::ApplyBlockStyle(nsCOMArray<nsIDOMNode>& arrayOfNodes, const nsA
     curParent = curNode->GetParentNode();
     offset = curParent ? curParent->IndexOf(curNode) : -1;
     nsAutoString curNodeTag;
-    curNode->Tag()->ToString(curNodeTag);
+    curNode->NodeInfo()->NameAtom()->ToString(curNodeTag);
     ToLowerCase(curNodeTag);
  
     // is it already the right kind of block?
@@ -7273,8 +7285,8 @@ nsHTMLEditRules::ApplyBlockStyle(nsCOMArray<nsIDOMNode>& arrayOfNodes, const nsA
              (curNodeTag.EqualsLiteral("td"))         ||
              nsHTMLEditUtils::IsList(curNode)                     ||
              (curNodeTag.EqualsLiteral("li"))         ||
-             curNode->Tag() == nsGkAtoms::blockquote  ||
-             curNode->Tag() == nsGkAtoms::div) {
+             curNode->IsAnyOfHTMLElements(nsGkAtoms::blockquote,
+                                          nsGkAtoms::div)) {
       curBlock = 0;  // forget any previous block used for previous inline nodes
       // recursion time
       nsCOMArray<nsIDOMNode> childArray;
@@ -7488,11 +7500,11 @@ nsHTMLEditRules::GetTopEnclosingMailCite(nsINode& aNode)
   nsCOMPtr<Element> ret;
 
   for (nsCOMPtr<nsINode> node = &aNode; node; node = node->GetParentNode()) {
-    if ((IsPlaintextEditor() && node->Tag() == nsGkAtoms::pre) ||
+    if ((IsPlaintextEditor() && node->IsHTMLElement(nsGkAtoms::pre)) ||
         nsHTMLEditUtils::IsMailCite(node)) {
       ret = node->AsElement();
     }
-    if (node->Tag() == nsGkAtoms::body) {
+    if (node->IsHTMLElement(nsGkAtoms::body)) {
       break;
     }
   }
@@ -7912,7 +7924,7 @@ nsHTMLEditRules::AdjustSelection(Selection* aSelection,
   if (nearNode && (nsTextEditUtils::IsBreak(nearNode) ||
                    nsEditor::IsTextNode(nearNode) ||
                    nsHTMLEditUtils::IsImage(nearNode) ||
-                   nearNode->Tag() == nsGkAtoms::hr)) {
+                   nearNode->IsHTMLElement(nsGkAtoms::hr))) {
     // this is a good place for the caret to be
     return NS_OK;
   }
@@ -7920,8 +7932,8 @@ nsHTMLEditRules::AdjustSelection(Selection* aSelection,
   nearNode = mHTMLEditor->GetNextHTMLNode(selNode, selOffset, true);
   if (nearNode && (nsTextEditUtils::IsBreak(nearNode) ||
                    nsEditor::IsTextNode(nearNode) ||
-                   nearNode->Tag() == nsGkAtoms::img ||
-                   nearNode->Tag() == nsGkAtoms::hr)) {
+                   nearNode->IsAnyOfHTMLElements(nsGkAtoms::img,
+                                                 nsGkAtoms::hr))) {
     return NS_OK; // this is a good place for the caret to be
   }
 
@@ -8106,18 +8118,18 @@ nsHTMLEditRules::RemoveEmptyNodes()
 
       if (node->IsElement()) {
         dom::Element* element = node->AsElement();
-        if (element->IsHTML(nsGkAtoms::body)) {
+        if (element->IsHTMLElement(nsGkAtoms::body)) {
           // don't delete the body
         } else if ((bIsMailCite = nsHTMLEditUtils::IsMailCite(element))  ||
-                   element->IsHTML(nsGkAtoms::a)                         ||
+                   element->IsHTMLElement(nsGkAtoms::a)                  ||
                    nsHTMLEditUtils::IsInlineStyle(element)               ||
                    nsHTMLEditUtils::IsList(element)                      ||
-                   element->IsHTML(nsGkAtoms::div)) {
+                   element->IsHTMLElement(nsGkAtoms::div)) {
           // only consider certain nodes to be empty for purposes of removal
           bIsCandidate = true;
         } else if (nsHTMLEditUtils::IsFormatNode(element) ||
                    nsHTMLEditUtils::IsListItem(element)   ||
-                   element->IsHTML(nsGkAtoms::blockquote)) {
+                   element->IsHTMLElement(nsGkAtoms::blockquote)) {
           // these node types are candidates if selection is not in them
           // if it is one of these, don't delete if selection inside.
           // this is so we can create empty headings, etc, for the
@@ -9041,7 +9053,7 @@ nsHTMLEditRules::RelativeChangeIndentationOfElementNode(nsIDOMNode *aNode, int8_
   // to unapply a CSS "indent" (<div style="margin-left: 40px;">) by
   // removing the DIV container instead of just removing the CSS property.
   nsCOMPtr<dom::Element> node = do_QueryInterface(aNode);
-  if (!node || !node->IsHTML(nsGkAtoms::div) ||
+  if (!node || !node->IsHTMLElement(nsGkAtoms::div) ||
       !mHTMLEditor ||
       node == mHTMLEditor->GetActiveEditingHost() ||
       !mHTMLEditor->IsDescendantOfEditorRoot(node) ||
@@ -9170,7 +9182,8 @@ nsHTMLEditRules::WillAbsolutePosition(Selection* aSelection,
       
       if (!curList || (sibling && sibling != GetAsDOMNode(curList))) {
         // create a new nested list of correct type
-        res = SplitAsNeeded(*curParent->Tag(), curParent, offset);
+        res = SplitAsNeeded(*curParent->NodeInfo()->NameAtom(), curParent,
+                            offset);
         NS_ENSURE_SUCCESS(res, res);
         if (!curPositionedDiv) {
           nsCOMPtr<nsINode> curParentParent = curParent->GetParentNode();
@@ -9182,8 +9195,8 @@ nsHTMLEditRules::WillAbsolutePosition(Selection* aSelection,
           mNewBlock = GetAsDOMNode(curPositionedDiv);
         }
         NS_ENSURE_STATE(mHTMLEditor);
-        curList = mHTMLEditor->CreateNode(curParent->Tag(), curPositionedDiv,
-                                          -1);
+        curList = mHTMLEditor->CreateNode(curParent->NodeInfo()->NameAtom(),
+                                          curPositionedDiv, -1);
         NS_ENSURE_STATE(curList);
         // curList is now the correct thing to put curNode in
         // remember our new block for postprocessing
@@ -9223,7 +9236,8 @@ nsHTMLEditRules::WillAbsolutePosition(Selection* aSelection,
          
         if (!curList || (sibling && sibling != GetAsDOMNode(curList))) {
           // create a new nested list of correct type
-          res = SplitAsNeeded(*curParent->Tag(), curParent, offset);
+          res = SplitAsNeeded(*curParent->NodeInfo()->NameAtom(), curParent,
+                              offset);
           NS_ENSURE_SUCCESS(res, res);
           if (!curPositionedDiv) {
             nsCOMPtr<nsINode> curParentParent = curParent->GetParentNode();
@@ -9236,8 +9250,8 @@ nsHTMLEditRules::WillAbsolutePosition(Selection* aSelection,
             mNewBlock = GetAsDOMNode(curPositionedDiv);
           }
           NS_ENSURE_STATE(mHTMLEditor);
-          curList = mHTMLEditor->CreateNode(curParent->Tag(), curPositionedDiv,
-                                            -1);
+          curList = mHTMLEditor->CreateNode(curParent->NodeInfo()->NameAtom(),
+                                            curPositionedDiv, -1);
           NS_ENSURE_STATE(curList);
         }
         NS_ENSURE_STATE(mHTMLEditor);
@@ -9253,7 +9267,7 @@ nsHTMLEditRules::WillAbsolutePosition(Selection* aSelection,
 
         if (!curPositionedDiv) 
         {
-          if (curNode->Tag() == nsGkAtoms::div) {
+          if (curNode->IsHTMLElement(nsGkAtoms::div)) {
             curPositionedDiv = curNode->AsElement();
             mNewBlock = GetAsDOMNode(curPositionedDiv);
             curList = nullptr;
diff --git a/editor/libeditor/nsHTMLEditUtils.cpp b/editor/libeditor/nsHTMLEditUtils.cpp
index ca7f430c07..d1f1e9473e 100644
--- a/editor/libeditor/nsHTMLEditUtils.cpp
+++ b/editor/libeditor/nsHTMLEditUtils.cpp
@@ -50,18 +50,17 @@ bool
 nsHTMLEditUtils::IsInlineStyle(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  nsIAtom* nodeAtom = aNode->Tag();
-  return (nodeAtom == nsGkAtoms::b)
-      || (nodeAtom == nsGkAtoms::i)
-      || (nodeAtom == nsGkAtoms::u)
-      || (nodeAtom == nsGkAtoms::tt)
-      || (nodeAtom == nsGkAtoms::s)
-      || (nodeAtom == nsGkAtoms::strike)
-      || (nodeAtom == nsGkAtoms::big)
-      || (nodeAtom == nsGkAtoms::small)
-      || (nodeAtom == nsGkAtoms::sub)
-      || (nodeAtom == nsGkAtoms::sup)
-      || (nodeAtom == nsGkAtoms::font);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::b,
+                                    nsGkAtoms::i,
+                                    nsGkAtoms::u,
+                                    nsGkAtoms::tt,
+                                    nsGkAtoms::s,
+                                    nsGkAtoms::strike,
+                                    nsGkAtoms::big,
+                                    nsGkAtoms::small,
+                                    nsGkAtoms::sub,
+                                    nsGkAtoms::sup,
+                                    nsGkAtoms::font);
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -79,16 +78,15 @@ bool
 nsHTMLEditUtils::IsFormatNode(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  nsIAtom* nodeAtom = aNode->Tag();
-  return (nodeAtom == nsGkAtoms::p)
-      || (nodeAtom == nsGkAtoms::pre)
-      || (nodeAtom == nsGkAtoms::h1)
-      || (nodeAtom == nsGkAtoms::h2)
-      || (nodeAtom == nsGkAtoms::h3)
-      || (nodeAtom == nsGkAtoms::h4)
-      || (nodeAtom == nsGkAtoms::h5)
-      || (nodeAtom == nsGkAtoms::h6)
-      || (nodeAtom == nsGkAtoms::address);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::p,
+                                    nsGkAtoms::pre,
+                                    nsGkAtoms::h1,
+                                    nsGkAtoms::h2,
+                                    nsGkAtoms::h3,
+                                    nsGkAtoms::h4,
+                                    nsGkAtoms::h5,
+                                    nsGkAtoms::h6,
+                                    nsGkAtoms::address);
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -127,13 +125,12 @@ nsHTMLEditUtils::IsSmall(nsIDOMNode* aNode)
 bool 
 nsHTMLEditUtils::IsHeader(nsINode& aNode)
 {
-  nsCOMPtr<nsIAtom> nodeAtom = aNode.Tag();
-  return (nodeAtom == nsGkAtoms::h1)
-      || (nodeAtom == nsGkAtoms::h2)
-      || (nodeAtom == nsGkAtoms::h3)
-      || (nodeAtom == nsGkAtoms::h4)
-      || (nodeAtom == nsGkAtoms::h5)
-      || (nodeAtom == nsGkAtoms::h6);
+  return aNode.IsAnyOfHTMLElements(nsGkAtoms::h1,
+                                   nsGkAtoms::h2,
+                                   nsGkAtoms::h3,
+                                   nsGkAtoms::h4,
+                                   nsGkAtoms::h5,
+                                   nsGkAtoms::h6);
 }
 
 bool
@@ -177,13 +174,12 @@ nsHTMLEditUtils::IsListItem(nsIDOMNode* aNode)
 }
 
 bool
-nsHTMLEditUtils::IsListItem(nsINode* node)
+nsHTMLEditUtils::IsListItem(nsINode* aNode)
 {
-  MOZ_ASSERT(node);
-  nsCOMPtr<nsIAtom> nodeAtom = node->Tag();
-  return (nodeAtom == nsGkAtoms::li)
-      || (nodeAtom == nsGkAtoms::dd)
-      || (nodeAtom == nsGkAtoms::dt);
+  MOZ_ASSERT(aNode);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::li,
+                                    nsGkAtoms::dd,
+                                    nsGkAtoms::dt);
 }
 
 
@@ -199,18 +195,17 @@ nsHTMLEditUtils::IsTableElement(nsIDOMNode* aNode)
 }
 
 bool
-nsHTMLEditUtils::IsTableElement(nsINode* node)
+nsHTMLEditUtils::IsTableElement(nsINode* aNode)
 {
-  MOZ_ASSERT(node);
-  nsCOMPtr<nsIAtom> nodeAtom = node->Tag();
-  return (nodeAtom == nsGkAtoms::table)
-      || (nodeAtom == nsGkAtoms::tr)
-      || (nodeAtom == nsGkAtoms::td)
-      || (nodeAtom == nsGkAtoms::th)
-      || (nodeAtom == nsGkAtoms::thead)
-      || (nodeAtom == nsGkAtoms::tfoot)
-      || (nodeAtom == nsGkAtoms::tbody)
-      || (nodeAtom == nsGkAtoms::caption);
+  MOZ_ASSERT(aNode);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::table,
+                                    nsGkAtoms::tr,
+                                    nsGkAtoms::td,
+                                    nsGkAtoms::th,
+                                    nsGkAtoms::thead,
+                                    nsGkAtoms::tfoot,
+                                    nsGkAtoms::tbody,
+                                    nsGkAtoms::caption);
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -228,14 +223,13 @@ bool
 nsHTMLEditUtils::IsTableElementButNotTable(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  nsCOMPtr<nsIAtom> nodeAtom = aNode->Tag();
-  return (nodeAtom == nsGkAtoms::tr)
-      || (nodeAtom == nsGkAtoms::td)
-      || (nodeAtom == nsGkAtoms::th)
-      || (nodeAtom == nsGkAtoms::thead)
-      || (nodeAtom == nsGkAtoms::tfoot)
-      || (nodeAtom == nsGkAtoms::tbody)
-      || (nodeAtom == nsGkAtoms::caption);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::tr,
+                                     nsGkAtoms::td,
+                                     nsGkAtoms::th,
+                                     nsGkAtoms::thead,
+                                     nsGkAtoms::tfoot,
+                                     nsGkAtoms::tbody,
+                                     nsGkAtoms::caption);
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -250,7 +244,7 @@ nsHTMLEditUtils::IsTable(nsIDOMNode* aNode)
 bool
 nsHTMLEditUtils::IsTable(nsINode* aNode)
 {
-  return aNode && aNode->IsElement() && aNode->Tag() == nsGkAtoms::table;
+  return aNode && aNode->IsHTMLElement(nsGkAtoms::table);
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -275,12 +269,10 @@ nsHTMLEditUtils::IsTableCell(nsIDOMNode* aNode)
 }
 
 bool
-nsHTMLEditUtils::IsTableCell(nsINode* node)
+nsHTMLEditUtils::IsTableCell(nsINode* aNode)
 {
-  MOZ_ASSERT(node);
-  nsCOMPtr<nsIAtom> nodeAtom = node->Tag();
-  return (nodeAtom == nsGkAtoms::td)
-      || (nodeAtom == nsGkAtoms::th);
+  MOZ_ASSERT(aNode);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th);
 }
 
 
@@ -310,13 +302,12 @@ nsHTMLEditUtils::IsList(nsIDOMNode* aNode)
 }
 
 bool
-nsHTMLEditUtils::IsList(nsINode* node)
+nsHTMLEditUtils::IsList(nsINode* aNode)
 {
-  MOZ_ASSERT(node);
-  nsCOMPtr<nsIAtom> nodeAtom = node->Tag();
-  return (nodeAtom == nsGkAtoms::ul)
-      || (nodeAtom == nsGkAtoms::ol)
-      || (nodeAtom == nsGkAtoms::dl);
+  MOZ_ASSERT(aNode);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::ul,
+                                    nsGkAtoms::ol,
+                                    nsGkAtoms::dl);
 }
 
 
@@ -366,7 +357,7 @@ nsHTMLEditUtils::IsPre(nsIDOMNode* aNode)
 bool
 nsHTMLEditUtils::IsImage(nsINode* aNode)
 {
-  return aNode && aNode->Tag() == nsGkAtoms::img;
+  return aNode && aNode->IsHTMLElement(nsGkAtoms::img);
 }
 
 bool 
@@ -409,7 +400,7 @@ bool
 nsHTMLEditUtils::IsNamedAnchor(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  if (!aNode->IsElement() || !aNode->AsElement()->IsHTML(nsGkAtoms::a)) {
+  if (!aNode->IsHTMLElement(nsGkAtoms::a)) {
     return false;
   }
 
@@ -443,7 +434,7 @@ bool
 nsHTMLEditUtils::IsMozDiv(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  return aNode->Tag() == nsGkAtoms::div &&
+  return aNode->IsHTMLElement(nsGkAtoms::div) &&
          nsTextEditUtils::HasMozAttr(GetAsDOMNode(aNode));
 }
 
@@ -499,15 +490,14 @@ bool
 nsHTMLEditUtils::IsFormWidget(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  nsCOMPtr<nsIAtom> nodeAtom = aNode->Tag();
-  return (nodeAtom == nsGkAtoms::textarea)
-      || (nodeAtom == nsGkAtoms::select)
-      || (nodeAtom == nsGkAtoms::button)
-      || (nodeAtom == nsGkAtoms::output)
-      || (nodeAtom == nsGkAtoms::keygen)
-      || (nodeAtom == nsGkAtoms::progress)
-      || (nodeAtom == nsGkAtoms::meter)
-      || (nodeAtom == nsGkAtoms::input);
+  return aNode->IsAnyOfHTMLElements(nsGkAtoms::textarea,
+                                    nsGkAtoms::select,
+                                    nsGkAtoms::button,
+                                    nsGkAtoms::output,
+                                    nsGkAtoms::keygen,
+                                    nsGkAtoms::progress,
+                                    nsGkAtoms::meter,
+                                    nsGkAtoms::input);
 }
 
 bool
diff --git a/editor/libeditor/nsHTMLEditor.cpp b/editor/libeditor/nsHTMLEditor.cpp
index 1ccc65ad83..b9145213e7 100644
--- a/editor/libeditor/nsHTMLEditor.cpp
+++ b/editor/libeditor/nsHTMLEditor.cpp
@@ -745,23 +745,20 @@ nsHTMLEditor::NodeIsBlockStatic(const dom::Element* aElement)
 {
   MOZ_ASSERT(aElement);
 
-  nsIAtom* tagAtom = aElement->Tag();
-  MOZ_ASSERT(tagAtom);
-
   // Nodes we know we want to treat as block
   // even though the parser says they're not:
-  if (tagAtom == nsGkAtoms::body ||
-      tagAtom == nsGkAtoms::head ||
-      tagAtom == nsGkAtoms::tbody ||
-      tagAtom == nsGkAtoms::thead ||
-      tagAtom == nsGkAtoms::tfoot ||
-      tagAtom == nsGkAtoms::tr ||
-      tagAtom == nsGkAtoms::th ||
-      tagAtom == nsGkAtoms::td ||
-      tagAtom == nsGkAtoms::li ||
-      tagAtom == nsGkAtoms::dt ||
-      tagAtom == nsGkAtoms::dd ||
-      tagAtom == nsGkAtoms::pre) {
+  if (aElement->IsAnyOfHTMLElements(nsGkAtoms::body,
+                                    nsGkAtoms::head,
+                                    nsGkAtoms::tbody,
+                                    nsGkAtoms::thead,
+                                    nsGkAtoms::tfoot,
+                                    nsGkAtoms::tr,
+                                    nsGkAtoms::th,
+                                    nsGkAtoms::td,
+                                    nsGkAtoms::li,
+                                    nsGkAtoms::dt,
+                                    nsGkAtoms::dd,
+                                    nsGkAtoms::pre)) {
     return true;
   }
 
@@ -771,11 +768,12 @@ nsHTMLEditor::NodeIsBlockStatic(const dom::Element* aElement)
   nsresult rv =
 #endif
     nsContentUtils::GetParserService()->
-    IsBlock(nsContentUtils::GetParserService()->HTMLAtomTagToId(tagAtom),
+    IsBlock(nsContentUtils::GetParserService()->HTMLAtomTagToId(
+              aElement->NodeInfo()->NameAtom()),
             isBlock);
   MOZ_ASSERT(rv == NS_OK);
 
-  AssertParserServiceIsCorrect(tagAtom, isBlock);
+  AssertParserServiceIsCorrect(aElement->NodeInfo()->NameAtom(), isBlock);
 
   return isBlock;
 }
@@ -1596,7 +1594,7 @@ nsHTMLEditor::InsertNodeAtPoint(nsIDOMNode *aNode,
   while (!CanContain(*parent, *node)) {
     // If the current parent is a root (body or table element)
     // then go no further - we can't insert
-    if (parent->Tag() == nsGkAtoms::body ||
+    if (parent->IsHTMLElement(nsGkAtoms::body) ||
         nsHTMLEditUtils::IsTableElement(parent)) {
       return NS_ERROR_FAILURE;
     }
@@ -1858,7 +1856,7 @@ nsHTMLEditor::GetHTMLBackgroundColorState(bool *aMixed, nsAString &aOutColor)
     }
 
     // Once we hit the body, we're done
-    if (element->IsHTML(nsGkAtoms::body)) {
+    if (element->IsHTMLElement(nsGkAtoms::body)) {
       return NS_OK;
     }
 
@@ -2297,7 +2295,7 @@ nsHTMLEditor::GetElementOrParentByTagName(const nsAString& aTagName,
     // stop at table cells, but that's too messy when you are trying to find
     // the parent table
     if (current->GetParentElement() &&
-        current->GetParentElement()->Tag() == nsGkAtoms::body) {
+        current->GetParentElement()->IsHTMLElement(nsGkAtoms::body)) {
       break;
     }
   }
@@ -3050,10 +3048,9 @@ nsHTMLEditor::GetEmbeddedObjects(nsISupportsArray** aNodeList)
 
       // See if it's an image or an embed and also include all links.
       // Let mail decide which link to send or not
-      if (element->IsHTML(nsGkAtoms::img) ||
-          element->IsHTML(nsGkAtoms::embed) ||
-          element->IsHTML(nsGkAtoms::a) ||
-          (element->IsHTML(nsGkAtoms::body) &&
+      if (element->IsAnyOfHTMLElements(nsGkAtoms::img, nsGkAtoms::embed,
+                                       nsGkAtoms::a) ||
+          (element->IsHTMLElement(nsGkAtoms::body) &&
            element->HasAttr(kNameSpaceID_None, nsGkAtoms::background))) {
         nsCOMPtr<nsIDOMNode> domNode = do_QueryInterface(node);
         (*aNodeList)->AppendElement(domNode);
@@ -3673,7 +3670,7 @@ bool
 nsHTMLEditor::SetCaretInTableCell(nsIDOMElement* aElement)
 {
   nsCOMPtr<dom::Element> element = do_QueryInterface(aElement);
-  if (!element || !element->IsHTML() ||
+  if (!element || !element->IsHTMLElement() ||
       !nsHTMLEditUtils::IsTableElement(element) ||
       !IsDescendantOfEditorRoot(element)) {
     return false;
@@ -4387,7 +4384,7 @@ nsHTMLEditor::IsEmptyNodeImpl(nsINode* aNode,
           break;
         }
 
-        if (aSingleBRDoesntCount && !*aSeenBR && child->IsHTML(nsGkAtoms::br)) {
+        if (aSingleBRDoesntCount && !*aSeenBR && child->IsHTMLElement(nsGkAtoms::br)) {
           // the first br in a block doesn't count if the caller so indicated
           *aSeenBR = true;
         } else {
@@ -4397,7 +4394,7 @@ nsHTMLEditor::IsEmptyNodeImpl(nsINode* aNode,
           if (child->IsElement()) {
             if (isListItemOrCell) {
               if (nsHTMLEditUtils::IsList(child) ||
-                  child->IsHTML(nsGkAtoms::table)) {
+                  child->IsHTMLElement(nsGkAtoms::table)) {
                 // break out if we find we aren't empty
                 *outIsEmptyNode = false;
                 return NS_OK;
@@ -4770,11 +4767,11 @@ nsHTMLEditor::AreNodesSameType(nsIContent* aNode1, nsIContent* aNode2)
   MOZ_ASSERT(aNode1);
   MOZ_ASSERT(aNode2);
 
-  if (aNode1->Tag() != aNode2->Tag()) {
+  if (aNode1->NodeInfo()->NameAtom() != aNode2->NodeInfo()->NameAtom()) {
     return false;
   }
 
-  if (!IsCSSEnabled() || !aNode1->IsHTML(nsGkAtoms::span)) {
+  if (!IsCSSEnabled() || !aNode1->IsHTMLElement(nsGkAtoms::span)) {
     return true;
   }
 
@@ -4824,13 +4821,14 @@ nsHTMLEditor::CopyLastEditableChildStyles(nsIDOMNode * aPreviousBlock, nsIDOMNod
   }
   while (childElement && (childElement->AsDOMNode() != aPreviousBlock)) {
     if (nsHTMLEditUtils::IsInlineStyle(childElement) ||
-        childElement->Tag() == nsGkAtoms::span) {
+        childElement->IsHTMLElement(nsGkAtoms::span)) {
       if (newStyles) {
-        newStyles = InsertContainerAbove(newStyles, childElement->Tag());
+        newStyles = InsertContainerAbove(newStyles,
+                                         childElement->NodeInfo()->NameAtom());
         NS_ENSURE_STATE(newStyles);
       } else {
-        deepestStyle = newStyles = CreateNode(childElement->Tag(), newBlock,
-                                              0);
+        deepestStyle = newStyles =
+          CreateNode(childElement->NodeInfo()->NameAtom(), newBlock, 0);
         NS_ENSURE_STATE(newStyles);
       }
       CloneAttributes(newStyles, childElement);
diff --git a/editor/libeditor/nsHTMLEditorStyle.cpp b/editor/libeditor/nsHTMLEditorStyle.cpp
index ddb636c3d7..f21893c11b 100644
--- a/editor/libeditor/nsHTMLEditorStyle.cpp
+++ b/editor/libeditor/nsHTMLEditorStyle.cpp
@@ -267,16 +267,19 @@ nsHTMLEditor::IsSimpleModifiableNode(nsIContent* aContent,
   }
 
   // First check for <b>, <i>, etc.
-  if (element->IsHTML(aProperty) && !element->GetAttrCount() &&
+  if (element->IsHTMLElement(aProperty) && !element->GetAttrCount() &&
       (!aAttribute || aAttribute->IsEmpty())) {
     return true;
   }
 
   // Special cases for various equivalencies: <strong>, <em>, <s>
   if (!element->GetAttrCount() &&
-      ((aProperty == nsGkAtoms::b && element->IsHTML(nsGkAtoms::strong)) ||
-       (aProperty == nsGkAtoms::i && element->IsHTML(nsGkAtoms::em)) ||
-       (aProperty == nsGkAtoms::strike && element->IsHTML(nsGkAtoms::s)))) {
+      ((aProperty == nsGkAtoms::b &&
+        element->IsHTMLElement(nsGkAtoms::strong)) ||
+       (aProperty == nsGkAtoms::i &&
+        element->IsHTMLElement(nsGkAtoms::em)) ||
+       (aProperty == nsGkAtoms::strike &&
+        element->IsHTMLElement(nsGkAtoms::s)))) {
     return true;
   }
 
@@ -286,7 +289,8 @@ nsHTMLEditor::IsSimpleModifiableNode(nsIContent* aContent,
     MOZ_ASSERT(atom);
 
     nsString attrValue;
-    if (element->IsHTML(aProperty) && IsOnlyAttribute(element, *aAttribute) &&
+    if (element->IsHTMLElement(aProperty) &&
+        IsOnlyAttribute(element, *aAttribute) &&
         element->GetAttr(kNameSpaceID_None, atom, attrValue) &&
         attrValue.Equals(*aValue, nsCaseInsensitiveStringComparator())) {
       // This is not quite correct, because it excludes cases like
@@ -300,7 +304,8 @@ nsHTMLEditor::IsSimpleModifiableNode(nsIContent* aContent,
   // attribute that sets only the style we're looking for, if this type of
   // style supports it
   if (!mHTMLCSSUtils->IsCSSEditableProperty(element, aProperty, aAttribute) ||
-      !element->IsHTML(nsGkAtoms::span) || element->GetAttrCount() != 1 ||
+      !element->IsHTMLElement(nsGkAtoms::span) ||
+      element->GetAttrCount() != 1 ||
       !element->HasAttr(kNameSpaceID_None, nsGkAtoms::style)) {
     return false;
   }
@@ -475,7 +480,7 @@ nsHTMLEditor::SetInlinePropertyOnNodeImpl(nsIContent* aNode,
     nsCOMPtr<dom::Element> tmp;
     // We only add style="" to <span>s with no attributes (bug 746515).  If we
     // don't have one, we need to make one.
-    if (aNode->IsElement() && aNode->AsElement()->IsHTML(nsGkAtoms::span) &&
+    if (aNode->IsHTMLElement(nsGkAtoms::span) &&
         !aNode->AsElement()->GetAttrCount()) {
       tmp = aNode->AsElement();
     } else {
@@ -493,7 +498,7 @@ nsHTMLEditor::SetInlinePropertyOnNodeImpl(nsIContent* aNode,
   }
 
   // is it already the right kind of node, but with wrong attribute?
-  if (aNode->Tag() == aProperty) {
+  if (aNode->IsHTMLElement(aProperty)) {
     // Just set the attribute on it.
     nsCOMPtr<nsIDOMElement> elem = do_QueryInterface(aNode);
     return SetAttribute(elem, *aAttribute, *aValue);
@@ -645,7 +650,7 @@ nsresult nsHTMLEditor::SplitStyleAbovePoint(nsCOMPtr<nsIDOMNode> *aNode,
         aProperty, aAttribute, isSet, firstValue, nsHTMLCSSUtils::eSpecified);
     }
     if (// node is the correct inline prop
-        (aProperty && node->Tag() == aProperty) ||
+        (aProperty && node->IsHTMLElement(aProperty)) ||
         // node is href - test if really <a href=...
         (aProperty == nsGkAtoms::href && nsHTMLEditUtils::IsLink(node)) ||
         // or node is any prop, and we asked to split them all
@@ -1684,13 +1689,13 @@ nsHTMLEditor::RelativeFontChangeOnTextNode( int32_t aSizeChange,
   // look for siblings that are correct type of node
   nsIAtom* nodeType = aSizeChange == 1 ? nsGkAtoms::big : nsGkAtoms::small;
   nsCOMPtr<nsIContent> sibling = GetPriorHTMLSibling(node);
-  if (sibling && sibling->Tag() == nodeType) {
+  if (sibling && sibling->IsHTMLElement(nodeType)) {
     // previous sib is already right kind of inline node; slide this over into it
     res = MoveNode(node, sibling, -1);
     return res;
   }
   sibling = GetNextHTMLSibling(node);
-  if (sibling && sibling->Tag() == nodeType) {
+  if (sibling && sibling->IsHTMLElement(nodeType)) {
     // following sib is already right kind of inline node; slide this over into it
     res = MoveNode(node, sibling, 0);
     return res;
@@ -1721,7 +1726,7 @@ nsHTMLEditor::RelativeFontChangeHelper(int32_t aSizeChange, nsINode* aNode)
   }
 
   // If this is a font node with size, put big/small inside it.
-  if (aNode->IsElement() && aNode->AsElement()->IsHTML(nsGkAtoms::font) &&
+  if (aNode->IsHTMLElement(nsGkAtoms::font) &&
       aNode->AsElement()->HasAttr(kNameSpaceID_None, nsGkAtoms::size)) {
     // Cycle through children and adjust relative font size.
     for (uint32_t i = aNode->GetChildCount(); i--; ) {
@@ -1761,9 +1766,8 @@ nsHTMLEditor::RelativeFontChangeOnNode(int32_t aSizeChange, nsIContent* aNode)
   }
   
   // Is it the opposite of what we want?
-  if (aNode->IsElement() &&
-      ((aSizeChange == 1 && aNode->AsElement()->IsHTML(nsGkAtoms::small)) ||
-       (aSizeChange == -1 && aNode->AsElement()->IsHTML(nsGkAtoms::big)))) {
+  if ((aSizeChange == 1 && aNode->IsHTMLElement(nsGkAtoms::small)) ||
+       (aSizeChange == -1 && aNode->IsHTMLElement(nsGkAtoms::big))) {
     // first populate any nested font tags that have the size attr set
     nsresult rv = RelativeFontChangeHelper(aSizeChange, aNode);
     NS_ENSURE_SUCCESS(rv, rv);
@@ -1781,13 +1785,13 @@ nsHTMLEditor::RelativeFontChangeOnNode(int32_t aSizeChange, nsIContent* aNode)
     // first look at siblings of aNode for matching bigs or smalls.
     // if we find one, move aNode into it.
     nsIContent* sibling = GetPriorHTMLSibling(aNode);
-    if (sibling && sibling->IsHTML(atom)) {
+    if (sibling && sibling->IsHTMLElement(atom)) {
       // previous sib is already right kind of inline node; slide this over into it
       return MoveNode(aNode, sibling, -1);
     }
 
     sibling = GetNextHTMLSibling(aNode);
-    if (sibling && sibling->IsHTML(atom)) {
+    if (sibling && sibling->IsHTMLElement(atom)) {
       // following sib is already right kind of inline node; slide this over into it
       return MoveNode(aNode, sibling, 0);
     }
@@ -1920,8 +1924,8 @@ nsHTMLEditor::RemoveElementIfNoStyleOrIdOrClass(nsIDOMNode* aElement)
   NS_ENSURE_TRUE(element, NS_ERROR_NULL_POINTER);
 
   // early way out if node is not the right kind of element
-  if ((!element->IsHTML(nsGkAtoms::span) &&
-       !element->IsHTML(nsGkAtoms::font)) ||
+  if ((!element->IsHTMLElement(nsGkAtoms::span) &&
+       !element->IsHTMLElement(nsGkAtoms::font)) ||
       HasStyleOrIdOrClass(element)) {
     return NS_OK;
   }
diff --git a/editor/libeditor/nsPlaintextEditor.cpp b/editor/libeditor/nsPlaintextEditor.cpp
index a76d4634fd..c5c8991064 100644
--- a/editor/libeditor/nsPlaintextEditor.cpp
+++ b/editor/libeditor/nsPlaintextEditor.cpp
@@ -1278,7 +1278,7 @@ nsPlaintextEditor::GetAndInitDocEncoder(const nsAString& aFormatType,
   {
     dom::Element* rootElement = GetRoot();
     NS_ENSURE_TRUE(rootElement, NS_ERROR_FAILURE);
-    if (!rootElement->IsHTML(nsGkAtoms::body)) {
+    if (!rootElement->IsHTMLElement(nsGkAtoms::body)) {
       rv = docEncoder->SetNativeContainerNode(rootElement);
       NS_ENSURE_SUCCESS(rv, rv);
     }
diff --git a/editor/libeditor/nsTableEditor.cpp b/editor/libeditor/nsTableEditor.cpp
index f9eeac4353..bf07bdebf5 100644
--- a/editor/libeditor/nsTableEditor.cpp
+++ b/editor/libeditor/nsTableEditor.cpp
@@ -219,18 +219,16 @@ nsHTMLEditor::GetFirstRow(nsIDOMElement* aTableElement, nsIDOMNode** aRowNode)
     nsCOMPtr<nsIContent> content = do_QueryInterface(tableChild);
     if (content)
     {
-      nsIAtom *atom = content->Tag();
-
-      if (atom == nsGkAtoms::tr) {
+      if (content->IsHTMLElement(nsGkAtoms::tr)) {
         // Found a row directly under <table>
         *aRowNode = tableChild;
         NS_ADDREF(*aRowNode);
         return NS_OK;
       }
       // Look for row in one of the row container elements      
-      if (atom == nsGkAtoms::tbody ||
-          atom == nsGkAtoms::thead ||
-          atom == nsGkAtoms::tfoot) {
+      if (content->IsAnyOfHTMLElements(nsGkAtoms::tbody,
+                                       nsGkAtoms::thead,
+                                       nsGkAtoms::tfoot)) {
         nsCOMPtr<nsIDOMNode> rowNode;
         res = tableChild->GetFirstChild(getter_AddRefs(rowNode));
         NS_ENSURE_SUCCESS(res, res);
@@ -3369,7 +3367,7 @@ nsHTMLEditor::IsEmptyCell(dom::Element* aCell)
 
   // We insert a single break into a cell by default
   //   to have some place to locate a cursor -- it is dispensable
-  if (cellChild->IsElement() && cellChild->AsElement()->IsHTML(nsGkAtoms::br)) {
+  if (cellChild->IsHTMLElement(nsGkAtoms::br)) {
     return true;
   }
 
diff --git a/editor/libeditor/nsTextEditRules.cpp b/editor/libeditor/nsTextEditRules.cpp
index 6d1c30df17..e77a7e7d46 100644
--- a/editor/libeditor/nsTextEditRules.cpp
+++ b/editor/libeditor/nsTextEditRules.cpp
@@ -1115,7 +1115,7 @@ nsTextEditRules::CreateTrailingBRIfNeeded()
   // assuming CreateBogusNodeIfNeeded() has been called first
   NS_ENSURE_TRUE(lastChild, NS_ERROR_NULL_POINTER);
 
-  if (!lastChild->IsHTML(nsGkAtoms::br)) {
+  if (!lastChild->IsHTMLElement(nsGkAtoms::br)) {
     nsAutoTxnsConserveSelection dontSpazMySelection(mEditor);
     nsCOMPtr<nsIDOMNode> domBody = do_QueryInterface(body);
     return CreateMozBR(domBody, body->Length());
diff --git a/editor/libeditor/nsTextEditUtils.cpp b/editor/libeditor/nsTextEditUtils.cpp
index 7d7051737e..67e41d978f 100644
--- a/editor/libeditor/nsTextEditUtils.cpp
+++ b/editor/libeditor/nsTextEditUtils.cpp
@@ -44,7 +44,7 @@ bool
 nsTextEditUtils::IsBreak(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  return aNode->IsElement() && aNode->AsElement()->IsHTML(nsGkAtoms::br);
+  return aNode->IsHTMLElement(nsGkAtoms::br);
 }
 
 
@@ -63,8 +63,7 @@ bool
 nsTextEditUtils::IsMozBR(nsINode* aNode)
 {
   MOZ_ASSERT(aNode);
-  return aNode->IsElement() &&
-         aNode->AsElement()->IsHTML(nsGkAtoms::br) &&
+  return aNode->IsHTMLElement(nsGkAtoms::br) &&
          aNode->AsElement()->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                                          NS_LITERAL_STRING("_moz"),
                                          eIgnoreCase);
diff --git a/editor/txtsvc/nsTextServicesDocument.cpp b/editor/txtsvc/nsTextServicesDocument.cpp
index 6bc3e93ab0..02747b018c 100644
--- a/editor/txtsvc/nsTextServicesDocument.cpp
+++ b/editor/txtsvc/nsTextServicesDocument.cpp
@@ -2248,7 +2248,7 @@ nsTextServicesDocument::IsBlockNode(nsIContent *aContent)
     return false;
   }
 
-  nsIAtom *atom = aContent->Tag();
+  nsIAtom *atom = aContent->NodeInfo()->NameAtom();
 
   return (sAAtom       != atom &&
           sAddressAtom != atom &&
diff --git a/embedding/browser/nsDocShellTreeOwner.cpp b/embedding/browser/nsDocShellTreeOwner.cpp
index 88283d9d4c..7ee08e787e 100644
--- a/embedding/browser/nsDocShellTreeOwner.cpp
+++ b/embedding/browser/nsDocShellTreeOwner.cpp
@@ -1002,7 +1002,7 @@ static bool
 UseSVGTitle(nsIDOMElement *currElement)
 {
   nsCOMPtr<dom::Element> element(do_QueryInterface(currElement));
-  if (!element || !element->IsSVG() || !element->GetParentNode())
+  if (!element || !element->IsSVGElement() || !element->GetParentNode())
     return false;
 
   return element->GetParentNode()->NodeType() != nsIDOMNode::DOCUMENT_NODE;
@@ -1051,10 +1051,9 @@ DefaultTooltipTextProvider::GetNodeText(nsIDOMNode *aNode, char16_t **aText,
     if (currElement) {
       nsCOMPtr<nsIContent> content(do_QueryInterface(currElement));
       if (content) {
-        nsIAtom *tagAtom = content->Tag();
-        if (tagAtom != mTag_dialog &&
-            tagAtom != mTag_dialogheader &&
-            tagAtom != mTag_window) {
+        if (!content->IsAnyOfXULElements(mTag_dialog,
+                                         mTag_dialogheader,
+                                         mTag_window)) {
           // first try the normal title attribute...
           currElement->GetAttribute(NS_LITERAL_STRING("title"), outText);
           if (outText.Length()) {
@@ -1079,7 +1078,7 @@ DefaultTooltipTextProvider::GetNodeText(nsIDOMNode *aNode, char16_t **aText,
                 uint32_t childNodeCount = childNodes->Length();
                 for (uint32_t i = 0; i < childNodeCount; i++) {
                   nsIContent* child = childNodes->Item(i);
-                  if (child->IsSVG(nsGkAtoms::title)) {
+                  if (child->IsSVGElement(nsGkAtoms::title)) {
                     static_cast<dom::SVGTitleElement*>(child)->GetTextContent(outText);
                     if (outText.Length())
                       found = true;
diff --git a/embedding/components/find/nsFind.cpp b/embedding/components/find/nsFind.cpp
index cafeb69808..1ca9743d37 100644
--- a/embedding/components/find/nsFind.cpp
+++ b/embedding/components/find/nsFind.cpp
@@ -744,19 +744,14 @@ nsFind::NextNode(nsIDOMRange* aSearchRange,
 
 bool nsFind::IsBlockNode(nsIContent* aContent)
 {
-  if (!aContent->IsHTML()) {
-    return false;
+  if (aContent->IsAnyOfHTMLElements(nsGkAtoms::img,
+                                    nsGkAtoms::hr,
+                                    nsGkAtoms::th,
+                                    nsGkAtoms::td)) {
+    return true;
   }
 
-  nsIAtom *atom = aContent->Tag();
-
-  if (atom == nsGkAtoms::img ||
-      atom == nsGkAtoms::hr ||
-      atom == nsGkAtoms::th ||
-      atom == nsGkAtoms::td)
-    return true;
-
-  return nsContentUtils::IsHTMLBlock(atom);
+  return nsContentUtils::IsHTMLBlock(aContent);
 }
 
 bool nsFind::IsTextNode(nsIDOMNode* aNode)
@@ -785,18 +780,13 @@ bool nsFind::IsVisibleNode(nsIDOMNode *aDOMNode)
 
 bool nsFind::SkipNode(nsIContent* aContent)
 {
-  nsIAtom *atom;
-
 #ifdef HAVE_BIDI_ITERATOR
-  atom = aContent->Tag();
-
   // We may not need to skip comment nodes,
   // now that IsTextNode distinguishes them from real text nodes.
   return (aContent->IsNodeOfType(nsINode::eCOMMENT) ||
-          (aContent->IsHTML() &&
-           (atom == sScriptAtom ||
-            atom == sNoframesAtom ||
-            atom == sSelectAtom)));
+          aContent->IsAnyOfHTMLElements(sScriptAtom,
+                                        sNoframesAtom,
+                                        sSelectAtom));
 
 #else /* HAVE_BIDI_ITERATOR */
   // Temporary: eventually we will have an iterator to do this,
@@ -807,13 +797,10 @@ bool nsFind::SkipNode(nsIContent* aContent)
   nsIContent *content = aContent;
   while (content)
   {
-    atom = content->Tag();
-
     if (aContent->IsNodeOfType(nsINode::eCOMMENT) ||
-        (content->IsHTML() &&
-         (atom == nsGkAtoms::script ||
-          atom == nsGkAtoms::noframes ||
-          atom == nsGkAtoms::select)))
+        content->IsAnyOfHTMLElements(nsGkAtoms::script,
+                                     nsGkAtoms::noframes,
+                                     nsGkAtoms::select))
     {
 #ifdef DEBUG_FIND
       printf("Skipping node: ");
diff --git a/embedding/components/webbrowserpersist/nsWebBrowserPersist.cpp b/embedding/components/webbrowserpersist/nsWebBrowserPersist.cpp
index e31075e210..c8627eada4 100644
--- a/embedding/components/webbrowserpersist/nsWebBrowserPersist.cpp
+++ b/embedding/components/webbrowserpersist/nsWebBrowserPersist.cpp
@@ -2648,7 +2648,7 @@ nsresult nsWebBrowserPersist::OnWalkDOMNode(nsIDOMNode *aNode)
         return NS_OK;
     }
 
-    if (content->IsSVG(nsGkAtoms::img))
+    if (content->IsSVGElement(nsGkAtoms::img))
     {
         StoreURIAttributeNS(aNode, "http://www.w3.org/1999/xlink", "href");
         return NS_OK;
@@ -2667,22 +2667,22 @@ nsresult nsWebBrowserPersist::OnWalkDOMNode(nsIDOMNode *aNode)
         return NS_OK;
     }
 
-    if (content->IsHTML(nsGkAtoms::body)) {
+    if (content->IsHTMLElement(nsGkAtoms::body)) {
         StoreURIAttribute(aNode, "background");
         return NS_OK;
     }
 
-    if (content->IsHTML(nsGkAtoms::table)) {
+    if (content->IsHTMLElement(nsGkAtoms::table)) {
         StoreURIAttribute(aNode, "background");
         return NS_OK;
     }
 
-    if (content->IsHTML(nsGkAtoms::tr)) {
+    if (content->IsHTMLElement(nsGkAtoms::tr)) {
         StoreURIAttribute(aNode, "background");
         return NS_OK;
     }
 
-    if (content->IsHTML(nsGkAtoms::td) || content->IsHTML(nsGkAtoms::th)) {
+    if (content->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th)) {
         StoreURIAttribute(aNode, "background");
         return NS_OK;
     }
@@ -2694,7 +2694,7 @@ nsresult nsWebBrowserPersist::OnWalkDOMNode(nsIDOMNode *aNode)
         return NS_OK;
     }
 
-    if (content->IsSVG(nsGkAtoms::script))
+    if (content->IsSVGElement(nsGkAtoms::script))
     {
         StoreURIAttributeNS(aNode, "http://www.w3.org/1999/xlink", "href");
         return NS_OK;
@@ -2950,7 +2950,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsHTML(nsGkAtoms::body)) {
+    if (content->IsHTMLElement(nsGkAtoms::body)) {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
         {
@@ -2959,7 +2959,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsHTML(nsGkAtoms::table)) {
+    if (content->IsHTMLElement(nsGkAtoms::table)) {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
         {
@@ -2968,7 +2968,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsHTML(nsGkAtoms::tr)) {
+    if (content->IsHTMLElement(nsGkAtoms::tr)) {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
         {
@@ -2977,7 +2977,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsHTML(nsGkAtoms::td) || content->IsHTML(nsGkAtoms::th)) {
+    if (content->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th)) {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
         {
@@ -3028,7 +3028,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsSVG(nsGkAtoms::img))
+    if (content->IsSVGElement(nsGkAtoms::img))
     {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
@@ -3056,7 +3056,7 @@ nsWebBrowserPersist::CloneNodeWithFixedUpAttributes(
         return rv;
     }
 
-    if (content->IsSVG(nsGkAtoms::script))
+    if (content->IsSVGElement(nsGkAtoms::script))
     {
         rv = GetNodeToFixup(aNodeIn, aNodeOut);
         if (NS_SUCCEEDED(rv) && *aNodeOut)
diff --git a/extensions/spellcheck/src/mozInlineSpellChecker.cpp b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
index 0f7b9259ea..496dd4df65 100644
--- a/extensions/spellcheck/src/mozInlineSpellChecker.cpp
+++ b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
@@ -1244,14 +1244,14 @@ mozInlineSpellChecker::ShouldSpellCheckNode(nsIEditor* aEditor,
   if (flags & nsIPlaintextEditor::eEditorMailMask) {
     nsIContent *parent = content->GetParent();
     while (parent) {
-      if (parent->IsHTML(nsGkAtoms::blockquote) &&
+      if (parent->IsHTMLElement(nsGkAtoms::blockquote) &&
           parent->AttrValueIs(kNameSpaceID_None,
                               nsGkAtoms::type,
                               nsGkAtoms::cite,
                               eIgnoreCase)) {
         return false;
       }
-      if (parent->IsHTML(nsGkAtoms::pre) &&
+      if (parent->IsHTMLElement(nsGkAtoms::pre) &&
           parent->AttrValueIs(kNameSpaceID_None,
                               nsGkAtoms::_class,
                               nsGkAtoms::mozsignature,
@@ -1285,7 +1285,7 @@ mozInlineSpellChecker::ShouldSpellCheckNode(nsIEditor* aEditor,
     // Get HTML element ancestor (might be aNode itself, although probably that
     // has to be a text node in real life here)
     nsIContent *parent = content;
-    while (!parent->IsHTML()) {
+    while (!parent->IsHTMLElement()) {
       parent = parent->GetParent();
       if (!parent) {
         return true;
diff --git a/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp b/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp
index 967de5da40..f63dbed309 100644
--- a/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp
+++ b/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp
@@ -366,8 +366,7 @@ IsDOMWordSeparator(char16_t ch)
 static inline bool
 IsBRElement(nsINode* aNode)
 {
-  return aNode->IsElement() &&
-         aNode->AsElement()->IsHTML(nsGkAtoms::br);
+  return aNode->IsHTMLElement(nsGkAtoms::br);
 }
 
 /**
@@ -442,7 +441,7 @@ IsBreakElement(nsINode* aNode)
 
   dom::Element *element = aNode->AsElement();
     
-  if (element->IsHTML(nsGkAtoms::br))
+  if (element->IsHTMLElement(nsGkAtoms::br))
     return true;
 
   // If we don't have a frame, we don't consider ourselves a break
diff --git a/image/SVGDocumentWrapper.cpp b/image/SVGDocumentWrapper.cpp
index 9a6bc7f801..cd42029e8b 100644
--- a/image/SVGDocumentWrapper.cpp
+++ b/image/SVGDocumentWrapper.cpp
@@ -410,7 +410,7 @@ SVGDocumentWrapper::GetRootSVGElem()
     return nullptr; // Can happen during destruction
 
   Element* rootElem = mViewer->GetDocument()->GetRootElement();
-  if (!rootElem || !rootElem->IsSVG(nsGkAtoms::svg)) {
+  if (!rootElem || !rootElem->IsSVGElement(nsGkAtoms::svg)) {
     return nullptr;
   }
 
diff --git a/layout/base/PositionedEventTargeting.cpp b/layout/base/PositionedEventTargeting.cpp
index 0819c1a39d..36c2267d7a 100644
--- a/layout/base/PositionedEventTargeting.cpp
+++ b/layout/base/PositionedEventTargeting.cpp
@@ -179,48 +179,46 @@ IsElementClickable(nsIFrame* aFrame, nsIAtom* stopAt = nullptr)
   // ancestors to look for elements accepting the click.
   for (nsIContent* content = aFrame->GetContent(); content;
        content = content->GetFlattenedTreeParent()) {
-    nsIAtom* tag = content->Tag();
-    if (content->IsHTML() && stopAt && tag == stopAt) {
+    if (stopAt && content->IsHTMLElement(stopAt)) {
       break;
     }
     if (HasTouchListener(content) || HasMouseListener(content)) {
       return true;
     }
-    if (content->IsHTML()) {
-      if (tag == nsGkAtoms::button ||
-          tag == nsGkAtoms::input ||
-          tag == nsGkAtoms::select ||
-          tag == nsGkAtoms::textarea ||
-          tag == nsGkAtoms::label) {
-        return true;
-      }
-      // Bug 921928: we don't have access to the content of remote iframe.
-      // So fluffing won't go there. We do an optimistic assumption here:
-      // that the content of the remote iframe needs to be a target.
-      if (tag == nsGkAtoms::iframe &&
-          content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::mozbrowser,
-                               nsGkAtoms::_true, eIgnoreCase) &&
-          content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::Remote,
-                               nsGkAtoms::_true, eIgnoreCase)) {
-        return true;
-      }
-    } else if (content->IsXUL()) {
-      nsIAtom* tag = content->Tag();
-      // See nsCSSFrameConstructor::FindXULTagData. This code is not
-      // really intended to be used with XUL, though.
-      if (tag == nsGkAtoms::button ||
-          tag == nsGkAtoms::checkbox ||
-          tag == nsGkAtoms::radio ||
-          tag == nsGkAtoms::autorepeatbutton ||
-          tag == nsGkAtoms::menu ||
-          tag == nsGkAtoms::menubutton ||
-          tag == nsGkAtoms::menuitem ||
-          tag == nsGkAtoms::menulist ||
-          tag == nsGkAtoms::scrollbarbutton ||
-          tag == nsGkAtoms::resizer) {
-        return true;
-      }
+    if (content->IsAnyOfHTMLElements(nsGkAtoms::button,
+                                     nsGkAtoms::input,
+                                     nsGkAtoms::select,
+                                     nsGkAtoms::textarea,
+                                     nsGkAtoms::label)) {
+      return true;
     }
+
+    // Bug 921928: we don't have access to the content of remote iframe.
+    // So fluffing won't go there. We do an optimistic assumption here:
+    // that the content of the remote iframe needs to be a target.
+    if (content->IsHTMLElement(nsGkAtoms::iframe) &&
+        content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::mozbrowser,
+                             nsGkAtoms::_true, eIgnoreCase) &&
+        content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::Remote,
+                             nsGkAtoms::_true, eIgnoreCase)) {
+      return true;
+    }
+
+    // See nsCSSFrameConstructor::FindXULTagData. This code is not
+    // really intended to be used with XUL, though.
+    if (content->IsAnyOfXULElements(nsGkAtoms::button,
+                                    nsGkAtoms::checkbox,
+                                    nsGkAtoms::radio,
+                                    nsGkAtoms::autorepeatbutton,
+                                    nsGkAtoms::menu,
+                                    nsGkAtoms::menubutton,
+                                    nsGkAtoms::menuitem,
+                                    nsGkAtoms::menulist,
+                                    nsGkAtoms::scrollbarbutton,
+                                    nsGkAtoms::resizer)) {
+      return true;
+    }
+
     static nsIContent::AttrValuesArray clickableRoles[] =
       { &nsGkAtoms::button, &nsGkAtoms::key, nullptr };
     if (content->FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::role,
diff --git a/layout/base/RestyleManager.cpp b/layout/base/RestyleManager.cpp
index 8e3c51f720..04c9c0a84f 100644
--- a/layout/base/RestyleManager.cpp
+++ b/layout/base/RestyleManager.cpp
@@ -267,7 +267,7 @@ DoApplyRenderingChangeToTree(nsIFrame* aFrame,
     if (aChange & nsChangeHint_UpdateTextPath) {
       if (aFrame->IsSVGText()) {
         // Invalidate and reflow the entire SVGTextFrame:
-        NS_ASSERTION(aFrame->GetContent()->IsSVG(nsGkAtoms::textPath),
+        NS_ASSERTION(aFrame->GetContent()->IsSVGElement(nsGkAtoms::textPath),
                      "expected frame for a <textPath> element");
         nsIFrame* text = nsLayoutUtils::GetClosestFrameOfType(
                                                       aFrame,
diff --git a/layout/base/RestyleManager.h b/layout/base/RestyleManager.h
index 015197111c..0aeb4c6757 100644
--- a/layout/base/RestyleManager.h
+++ b/layout/base/RestyleManager.h
@@ -184,10 +184,10 @@ public:
       if (pseudoType == nsCSSPseudoElements::ePseudo_NotPseudoElement) {
         mElementContexts.Put(aContent, aStyleContext);
       } else if (pseudoType == nsCSSPseudoElements::ePseudo_before) {
-        MOZ_ASSERT(aContent->Tag() == nsGkAtoms::mozgeneratedcontentbefore);
+        MOZ_ASSERT(aContent->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentbefore);
         mBeforePseudoContexts.Put(aContent->GetParent(), aStyleContext);
       } else if (pseudoType == nsCSSPseudoElements::ePseudo_after) {
-        MOZ_ASSERT(aContent->Tag() == nsGkAtoms::mozgeneratedcontentafter);
+        MOZ_ASSERT(aContent->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentafter);
         mAfterPseudoContexts.Put(aContent->GetParent(), aStyleContext);
       }
     }
@@ -199,11 +199,11 @@ public:
         return mElementContexts.GetWeak(aContent);
       }
       if (aPseudoType == nsCSSPseudoElements::ePseudo_before) {
-        MOZ_ASSERT(aContent->Tag() == nsGkAtoms::mozgeneratedcontentbefore);
+        MOZ_ASSERT(aContent->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentbefore);
         return mBeforePseudoContexts.GetWeak(aContent->GetParent());
       }
       if (aPseudoType == nsCSSPseudoElements::ePseudo_after) {
-        MOZ_ASSERT(aContent->Tag() == nsGkAtoms::mozgeneratedcontentafter);
+        MOZ_ASSERT(aContent->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentafter);
         return mAfterPseudoContexts.GetWeak(aContent->GetParent());
       }
       MOZ_ASSERT(false, "unexpected aPseudoType");
diff --git a/layout/base/RestyleTracker.cpp b/layout/base/RestyleTracker.cpp
index 9ce0b47329..6d0413733a 100644
--- a/layout/base/RestyleTracker.cpp
+++ b/layout/base/RestyleTracker.cpp
@@ -40,7 +40,7 @@ FrameTagToString(dom::Element* aElement)
     nsFrame::ListTag(result, frame);
   } else {
     nsAutoString buf;
-    aElement->Tag()->ToString(buf);
+    aElement->NodeInfo()->NameAtom()->ToString(buf);
     result.AppendPrintf("(%s@%p)", NS_ConvertUTF16toUTF8(buf).get(), aElement);
   }
   return result;
diff --git a/layout/base/nsBidiPresUtils.cpp b/layout/base/nsBidiPresUtils.cpp
index 261782c79d..5eaa96ae3f 100644
--- a/layout/base/nsBidiPresUtils.cpp
+++ b/layout/base/nsBidiPresUtils.cpp
@@ -99,7 +99,7 @@ struct BidiParagraphData {
       for (nsIContent* content = aBlockFrame->GetContent() ; content; 
            content = content->GetParent()) {
         if (content->IsNodeOfType(nsINode::eHTML_FORM_CONTROL) ||
-            content->IsXUL()) {
+            content->IsXULElement()) {
           mIsVisual = false;
           break;
         }
@@ -1158,7 +1158,7 @@ nsBidiPresUtils::TraverseFrames(nsBlockFrame*              aBlockFrame,
         // U+FFFC"
         // <wbr>, however, is treated as U+200B ZERO WIDTH SPACE. See
         // http://dev.w3.org/html5/spec/Overview.html#phrasing-content-1
-        aBpd->AppendUnichar(content->IsHTML(nsGkAtoms::wbr) ?
+        aBpd->AppendUnichar(content->IsHTMLElement(nsGkAtoms::wbr) ?
                             kZWSP : kObjectSubstitute);
         if (!frame->IsInlineOutside()) {
           // if it is not inline, end the paragraph
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 80c727508e..fafaf5abaa 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -1625,14 +1625,14 @@ nsCSSFrameConstructor::CreateGeneratedContent(nsFrameConstructorState& aState,
         attrNameSpace = nameSpaceVal.ToInteger(&error);
         contentString.Cut(0, barIndex + 1);
         if (contentString.Length()) {
-          if (mDocument->IsHTML() && aParentContent->IsHTML()) {
+          if (mDocument->IsHTMLDocument() && aParentContent->IsHTMLElement()) {
             ToLowerCase(contentString);
           }
           attrName = do_GetAtom(contentString);
         }
       }
       else {
-        if (mDocument->IsHTML() && aParentContent->IsHTML()) {
+        if (mDocument->IsHTMLDocument() && aParentContent->IsHTMLElement()) {
           ToLowerCase(contentString);
         }
         attrName = do_GetAtom(contentString);
@@ -1700,7 +1700,7 @@ nsCSSFrameConstructor::CreateGeneratedContent(nsFrameConstructorState& aState,
         return content.forget();
       }
 
-      if (aParentContent->IsHTML() &&
+      if (aParentContent->IsHTMLElement() &&
           aParentContent->NodeInfo()->Equals(nsGkAtoms::input)) {
         if (aParentContent->HasAttr(kNameSpaceID_None, nsGkAtoms::value)) {
           nsCOMPtr<nsIContent> content;
@@ -2370,7 +2370,7 @@ nsCSSFrameConstructor::PropagateScrollToViewport()
   // of the viewport
   // XXX what about XHTML?
   nsCOMPtr<nsIDOMHTMLDocument> htmlDoc(do_QueryInterface(mDocument));
-  if (!htmlDoc || !docElement->IsHTML()) {
+  if (!htmlDoc || !docElement->IsHTMLElement()) {
     return nullptr;
   }
 
@@ -2537,7 +2537,7 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
 
   // Check whether we need to build a XUL box or SVG root frame
 #ifdef MOZ_XUL
-  if (aDocElement->IsXUL()) {
+  if (aDocElement->IsXULElement()) {
     contentFrame = NS_NewDocElementBoxFrame(mPresShell, styleContext);
     InitAndRestoreFrame(state, aDocElement, mDocElementContainingBlock,
                         contentFrame);
@@ -2546,8 +2546,8 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
   }
   else
 #endif
-  if (aDocElement->IsSVG()) {
-    if (aDocElement->Tag() != nsGkAtoms::svg) {
+  if (aDocElement->IsSVGElement()) {
+    if (!aDocElement->IsSVGElement(nsGkAtoms::svg)) {
       return nullptr;
     }
     // We're going to call the right function ourselves, so no need to give a
@@ -2562,8 +2562,9 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
     already_AddRefed<nsStyleContext> extraRef =
       nsRefPtr<nsStyleContext>(styleContext).forget();
     FrameConstructionItem item(&rootSVGData, aDocElement,
-                               aDocElement->Tag(), kNameSpaceID_SVG,
-                               nullptr, extraRef, true, nullptr);
+                               aDocElement->NodeInfo()->NameAtom(),
+                               kNameSpaceID_SVG, nullptr, extraRef, true,
+                               nullptr);
 
     nsFrameItems frameItems;
     contentFrame = static_cast<nsContainerFrame*>(
@@ -2597,8 +2598,9 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
     already_AddRefed<nsStyleContext> extraRef =
       nsRefPtr<nsStyleContext>(styleContext).forget();
     FrameConstructionItem item(&rootTableData, aDocElement,
-                               aDocElement->Tag(), kNameSpaceID_None,
-                               nullptr, extraRef, true, nullptr);
+                               aDocElement->NodeInfo()->NameAtom(),
+                               kNameSpaceID_None, nullptr, extraRef, true,
+                               nullptr);
 
     nsFrameItems frameItems;
     // if the document is a table then just populate it.
@@ -2795,7 +2797,7 @@ nsCSSFrameConstructor::SetUpDocElementContainingBlock(nsIContent* aDocElement)
 
   if (!isPaginated) {
 #ifdef MOZ_XUL
-    if (aDocElement->IsXUL())
+    if (aDocElement->IsXULElement())
     {
       // pass a temporary stylecontext, the correct one will be set later
       rootFrame = NS_NewRootBoxFrame(mPresShell, viewportPseudoStyle);
@@ -2824,11 +2826,11 @@ nsCSSFrameConstructor::SetUpDocElementContainingBlock(nsIContent* aDocElement)
   // will act as the scrolling mechanism for the viewport.
   // XXX Do we even need a viewport when printing to a printer?
 
-  bool isHTML = aDocElement->IsHTML();
+  bool isHTML = aDocElement->IsHTMLElement();
   bool isXUL = false;
 
   if (!isHTML) {
-    isXUL = aDocElement->IsXUL();
+    isXUL = aDocElement->IsXULElement();
   }
 
   // Never create scrollbars for XUL documents
@@ -3502,7 +3504,7 @@ nsCSSFrameConstructor::FindHTMLData(Element* aElement,
       (!aParentFrame ||
        !IsFrameForFieldSet(aParentFrame, aParentFrame->GetType()) ||
        !aElement->GetParent() ||
-       !aElement->GetParent()->IsHTML(nsGkAtoms::fieldset) ||
+       !aElement->GetParent()->IsHTMLElement(nsGkAtoms::fieldset) ||
        aStyleContext->StyleDisplay()->IsFloatingStyle() ||
        aStyleContext->StyleDisplay()->IsAbsolutelyPositionedStyle())) {
     // <legend> is only special inside fieldset, check both the frame tree
@@ -3727,8 +3729,8 @@ nsCSSFrameConstructor::ConstructFrameFromItemInternal(FrameConstructionItem& aIt
                "Unexpected FCDATA_FORCED_NON_SCROLLABLE_BLOCK flag");
 
   // Don't create a subdocument frame for iframes if we're creating extra frames
-  if (aState.mCreatingExtraFrames && aItem.mContent->IsHTML() &&
-      aItem.mContent->Tag() == nsGkAtoms::iframe)
+  if (aState.mCreatingExtraFrames &&
+      aItem.mContent->IsHTMLElement(nsGkAtoms::iframe))
   {
     return;
   }
@@ -4598,7 +4600,7 @@ nsCSSFrameConstructor::FindDisplayData(const nsStyleDisplay* aDisplay,
   // XXXbz is this the right place to do this?  If this code moves,
   // make this function static.
   bool propagatedScrollToViewport = false;
-  if (aElement->IsHTML(nsGkAtoms::body)) {
+  if (aElement->IsHTMLElement(nsGkAtoms::body)) {
     propagatedScrollToViewport =
       PropagateScrollToViewport() == aElement;
   }
@@ -5501,7 +5503,8 @@ nsCSSFrameConstructor::DoAddFrameConstructionItems(nsFrameConstructorState& aSta
     }
   }
   AddFrameConstructionItemsInternal(aState, aContent, aParentFrame,
-                                    aContent->Tag(), aContent->GetNameSpaceID(),
+                                    aContent->NodeInfo()->NameAtom(),
+                                    aContent->GetNameSpaceID(),
                                     aSuppressWhiteSpaceOptimizations,
                                     aStyleContext,
                                     flags, aAnonChildren,
@@ -5565,7 +5568,7 @@ nsCSSFrameConstructor::AddFrameConstructionItemsInternal(nsFrameConstructorState
                   aContent->IsElement(),
                   "Shouldn't get anything else here!");
   MOZ_ASSERT(!aContent->GetPrimaryFrame() || aState.mCreatingExtraFrames ||
-             aContent->Tag() == nsGkAtoms::area);
+             aContent->NodeInfo()->NameAtom() == nsGkAtoms::area);
 
   // The following code allows the user to specify the base tag
   // of an element using XBL.  XUL and HTML objects (like boxes, menus, etc.)
@@ -5625,14 +5628,12 @@ nsCSSFrameConstructor::AddFrameConstructionItemsInternal(nsFrameConstructorState
   nsIContent *parent = aContent->GetParent();
   if (parent) {
     // Check tag first, since that check will usually fail
-    nsIAtom* parentTag = parent->Tag();
-    if ((parentTag == nsGkAtoms::select || parentTag == nsGkAtoms::optgroup) &&
-        parent->IsHTML() &&
+    if (parent->IsAnyOfHTMLElements(nsGkAtoms::select, nsGkAtoms::optgroup) &&
         // <option> is ok no matter what
-        !aContent->IsHTML(nsGkAtoms::option) &&
+        !aContent->IsHTMLElement(nsGkAtoms::option) &&
         // <optgroup> is OK in <select> but not in <optgroup>
-        (!aContent->IsHTML(nsGkAtoms::optgroup) ||
-         parentTag != nsGkAtoms::select) &&
+        (!aContent->IsHTMLElement(nsGkAtoms::optgroup) ||
+         !parent->IsHTMLElement(nsGkAtoms::select)) &&
         // Allow native anonymous content no matter what
         !aContent->IsRootOfNativeAnonymousSubtree()) {
       // No frame for aContent
@@ -6224,7 +6225,7 @@ AdjustAppendParentForAfterContent(nsFrameManager* aFrameManager,
         nsIContent* c = child->GetContent();
         if (child->IsGeneratedContentFrame()) {
           nsIContent* p = c->GetParent();
-          if (c->Tag() == nsGkAtoms::mozgeneratedcontentafter) {
+          if (c->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentafter) {
             if (!nsContentUtils::ContentIsDescendantOf(aChild, p) &&
                 p != aContainer &&
                 nsContentUtils::PositionIsBefore(p, aChild)) {
@@ -6504,7 +6505,7 @@ nsCSSFrameConstructor::IsValidSibling(nsIFrame*              aSibling,
       aSibling = cif;
     }
     nsIAtom* sibType = aSibling->GetType();
-    bool legendContent = aContent->IsHTML(nsGkAtoms::legend);
+    bool legendContent = aContent->IsHTMLElement(nsGkAtoms::legend);
 
     if ((legendContent  && (nsGkAtoms::legendFrame != sibType)) ||
         (!legendContent && (nsGkAtoms::legendFrame == sibType)))
@@ -6638,7 +6639,7 @@ GetAdjustedParentFrame(nsContainerFrame* aParentFrame,
   if (nsGkAtoms::fieldSetFrame == aParentFrameType) {
     // If the parent is a fieldSet, use the fieldSet's area frame as the
     // parent unless the new content is a legend.
-    if (!aChildContent->IsHTML(nsGkAtoms::legend)) {
+    if (!aChildContent->IsHTMLElement(nsGkAtoms::legend)) {
       newParent = GetFieldSetBlockFrame(aParentFrame);
     }
   }
@@ -6800,9 +6801,7 @@ static bool
 IsSpecialFramesetChild(nsIContent* aContent)
 {
   // IMPORTANT: This must match the conditions in nsHTMLFramesetFrame::Init.
-  return aContent->IsHTML() &&
-    (aContent->Tag() == nsGkAtoms::frameset ||
-     aContent->Tag() == nsGkAtoms::frame);
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::frameset, nsGkAtoms::frame);
 }
 
 static void
@@ -6814,7 +6813,7 @@ static
 bool
 IsXULListBox(nsIContent* aContainer)
 {
-  return (aContainer->IsXUL() && aContainer->Tag() == nsGkAtoms::listbox);
+  return (aContainer->IsXULElement(nsGkAtoms::listbox));
 }
 
 static
@@ -6825,7 +6824,7 @@ MaybeGetListBoxBodyFrame(nsIContent* aContainer, nsIContent* aChild)
     return nullptr;
 
   if (IsXULListBox(aContainer) &&
-      aChild->IsXUL() && aChild->Tag() == nsGkAtoms::listitem) {
+      aChild->IsXULElement(nsGkAtoms::listitem)) {
     nsCOMPtr<nsIDOMXULElement> xulElement = do_QueryInterface(aContainer);
     nsCOMPtr<nsIBoxObject> boxObject;
     xulElement->GetBoxObject(getter_AddRefs(boxObject));
@@ -6883,7 +6882,7 @@ nsCSSFrameConstructor::MaybeConstructLazily(Operation aOperation,
                                             nsIContent* aChild)
 {
   if (mPresShell->GetPresContext()->IsChrome() || !aContainer ||
-      aContainer->IsInNativeAnonymousSubtree() || aContainer->IsXUL()) {
+      aContainer->IsInNativeAnonymousSubtree() || aContainer->IsXULElement()) {
     return false;
   }
 
@@ -6891,7 +6890,7 @@ nsCSSFrameConstructor::MaybeConstructLazily(Operation aOperation,
     if (aChild->IsRootOfAnonymousSubtree() ||
         (aChild->HasFlag(NODE_IS_IN_SHADOW_TREE) &&
          !aChild->IsInNativeAnonymousSubtree()) ||
-        aChild->IsEditable() || aChild->IsXUL()) {
+        aChild->IsEditable() || aChild->IsXULElement()) {
       return false;
     }
   } else { // CONTENTAPPEND
@@ -6900,7 +6899,7 @@ nsCSSFrameConstructor::MaybeConstructLazily(Operation aOperation,
     for (nsIContent* child = aChild; child; child = child->GetNextSibling()) {
       NS_ASSERTION(!child->IsRootOfAnonymousSubtree(),
                    "Should be coming through the CONTENTAPPEND case");
-      if (child->IsXUL() || child->IsEditable()) {
+      if (child->IsXULElement() || child->IsEditable()) {
         return false;
       }
     }
@@ -7730,7 +7729,7 @@ nsCSSFrameConstructor::ContentRangeInserted(nsIContent*            aContainer,
   NS_ASSERTION(isSingleInsert || frameType != nsGkAtoms::fieldSetFrame,
                "Unexpected parent");
   if (IsFrameForFieldSet(insertion.mParentFrame, frameType) &&
-      aStartChild->Tag() == nsGkAtoms::legend) {
+      aStartChild->NodeInfo()->NameAtom() == nsGkAtoms::legend) {
     // Just reframe the parent, since figuring out whether this
     // should be the new legend and then handling it is too complex.
     // We could do a little better here --- check if the fieldset already
@@ -8452,8 +8451,7 @@ InvalidateCanvasIfNeeded(nsIPresShell* presShell, nsIContent* node)
     }
 
     // Check whether it's an HTML body
-    if (node->Tag() != nsGkAtoms::body ||
-        !node->IsHTML()) {
+    if (!node->IsHTMLElement(nsGkAtoms::body)) {
       return;
     }
   }
@@ -9014,7 +9012,7 @@ nsCSSFrameConstructor::ReplicateFixedFrames(nsPageContentFrame* aParentFrame)
         nsLayoutUtils::GetStyleFrame(content)->StyleContext();
       FrameConstructionItemList items;
       AddFrameConstructionItemsInternal(state, content, canvasFrame,
-                                        content->Tag(),
+                                        content->NodeInfo()->NameAtom(),
                                         content->GetNameSpaceID(),
                                         true,
                                         styleContext,
@@ -9085,7 +9083,7 @@ nsCSSFrameConstructor::GetInsertionPoint(nsIContent* aContainer,
                            insertionElement);
 
   // Fieldsets have multiple insertion points.
-  if (insertionElement->IsHTML(nsGkAtoms::fieldset)) {
+  if (insertionElement->IsHTMLElement(nsGkAtoms::fieldset)) {
     insertion.mMultiple = true;
   }
 
@@ -10471,7 +10469,8 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
                      ITEM_IS_ANONYMOUSCONTENTCREATOR_CONTENT | aExtraFlags;
 
     AddFrameConstructionItemsInternal(aState, content, aFrame,
-                                      content->Tag(), content->GetNameSpaceID(),
+                                      content->NodeInfo()->NameAtom(),
+                                      content->GetNameSpaceID(),
                                       true, styleContext, flags,
                                       anonChildren, aItemsToConstruct);
   }
@@ -10647,7 +10646,8 @@ nsCSSFrameConstructor::ProcessChildren(nsFrameConstructorState& aState,
     if (!aFrame->IsGeneratedContentFrame() &&
         mPresShell->GetPresContext()->IsChrome()) {
       nsIContent *badKid = AnyKidsNeedBlockParent(aFrameItems.FirstChild());
-      nsDependentAtomString parentTag(aContent->Tag()), kidTag(badKid->Tag());
+      nsDependentAtomString parentTag(aContent->NodeInfo()->NameAtom()),
+                            kidTag(badKid->NodeInfo()->NameAtom());
       const char16_t* params[] = { parentTag.get(), kidTag.get() };
       const nsStyleDisplay *display = frameStyleContext->StyleDisplay();
       const char *message =
@@ -11532,7 +11532,8 @@ nsCSSFrameConstructor::CreateListBoxContent(nsPresContext*         aPresContext,
 
     FrameConstructionItemList items;
     AddFrameConstructionItemsInternal(state, aChild, aParentFrame,
-                                      aChild->Tag(), aChild->GetNameSpaceID(),
+                                      aChild->NodeInfo()->NameAtom(),
+                                      aChild->GetNameSpaceID(),
                                       true, styleContext,
                                       ITEM_ALLOW_XBL_BASE, nullptr, items);
     ConstructFramesFromItemList(state, items, aParentFrame, frameItems);
@@ -11927,7 +11928,8 @@ nsCSSFrameConstructor::BuildInlineChildItems(nsFrameConstructorState& aState,
       nsRefPtr<nsStyleContext> childContext =
         ResolveStyleContext(parentStyleContext, content, &aState);
 
-      AddFrameConstructionItemsInternal(aState, content, nullptr, content->Tag(),
+      AddFrameConstructionItemsInternal(aState, content, nullptr,
+                                        content->NodeInfo()->NameAtom(),
                                         content->GetNameSpaceID(),
                                         iter.XBLInvolved(), childContext,
                                         flags, nullptr,
diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index c8ff2a6cc1..10ccdff960 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -1162,7 +1162,7 @@ FindElementBackground(nsIFrame* aForFrame, nsIFrame* aRootElementFrame,
   // was propagated to the viewport.
 
   nsIContent* content = aForFrame->GetContent();
-  if (!content || content->Tag() != nsGkAtoms::body)
+  if (!content || content->NodeInfo()->NameAtom() != nsGkAtoms::body)
     return true; // not frame for a "body" element
   // It could be a non-HTML "body" element but that's OK, we'd fail the
   // bodyContent check below
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index 020706731d..75a3365405 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -6038,7 +6038,7 @@ nsDisplaySVGEffects::BuildLayer(nsDisplayListBuilder* aBuilder,
   bool hasSVGLayout = (mFrame->GetStateBits() & NS_FRAME_SVG_LAYOUT);
   if (hasSVGLayout) {
     nsISVGChildFrame *svgChildFrame = do_QueryFrame(mFrame);
-    if (!svgChildFrame || !mFrame->GetContent()->IsSVG()) {
+    if (!svgChildFrame || !mFrame->GetContent()->IsSVGElement()) {
       NS_ASSERTION(false, "why?");
       return nullptr;
     }
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 9e703590e0..767c35a892 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -1000,7 +1000,7 @@ nsDocumentViewer::LoadComplete(nsresult aStatus)
 
     docShell->GetRestoringDocument(&restoring);
     if (!restoring) {
-      NS_ASSERTION(mDocument->IsXUL() || // readyState for XUL is bogus
+      NS_ASSERTION(mDocument->IsXULDocument() || // readyState for XUL is bogus
                    mDocument->GetReadyStateEnum() ==
                      nsIDocument::READYSTATE_INTERACTIVE ||
                    // test_stricttransportsecurity.html has old-style
@@ -2284,7 +2284,7 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
   // The document will fill in the document sheets when we create the presshell
 
   if (aDocument->IsBeingUsedAsImage()) {
-    MOZ_ASSERT(aDocument->IsSVG(),
+    MOZ_ASSERT(aDocument->IsSVGDocument(),
                "Do we want to skip most sheets for this new image type?");
 
     // SVG-as-an-image must be kept as light and small as possible. We
@@ -2364,7 +2364,7 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
     styleSet->PrependStyleSheet(nsStyleSet::eOverrideSheet, sheet);
   }
 
-  if (!aDocument->IsSVG()) {
+  if (!aDocument->IsSVGDocument()) {
     // !!! IMPORTANT - KEEP THIS BLOCK IN SYNC WITH
     // !!! SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded.
 
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 15319682ef..0fedb1abbb 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -1188,7 +1188,7 @@ nsLayoutUtils::GetBeforeFrameForContent(nsIFrame* aFrame,
     const nsTArray<nsIContent*>& pseudos(*prop);
     for (uint32_t i = 0; i < pseudos.Length(); ++i) {
       if (pseudos[i]->GetParent() == aContent &&
-          pseudos[i]->Tag() == nsGkAtoms::mozgeneratedcontentbefore) {
+          pseudos[i]->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentbefore) {
         return pseudos[i]->GetPrimaryFrame();
       }
     }
@@ -1227,7 +1227,7 @@ nsLayoutUtils::GetAfterFrameForContent(nsIFrame* aFrame,
     const nsTArray<nsIContent*>& pseudos(*prop);
     for (uint32_t i = 0; i < pseudos.Length(); ++i) {
       if (pseudos[i]->GetParent() == aContent &&
-          pseudos[i]->Tag() == nsGkAtoms::mozgeneratedcontentafter) {
+          pseudos[i]->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentafter) {
         return pseudos[i]->GetPrimaryFrame();
       }
     }
@@ -1332,7 +1332,7 @@ nsLayoutUtils::IsGeneratedContentFor(nsIContent* aContent,
     return false;
   }
 
-  return (aFrame->GetContent()->Tag() == nsGkAtoms::mozgeneratedcontentbefore) ==
+  return (aFrame->GetContent()->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentbefore) ==
     (aPseudoElement == nsCSSPseudoElements::before);
 }
 
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 99567ca2ed..da0f37fb31 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -1008,7 +1008,7 @@ nsPresContext::Init(nsDeviceContext* aDeviceContext)
         if (parentItem) {
           Element* containingElement =
             parent->FindContentForSubDocument(mDocument);
-          if (!containingElement->IsXUL() ||
+          if (!containingElement->IsXULElement() ||
               !containingElement->
                 HasAttr(kNameSpaceID_None,
                         nsGkAtoms::forceOwnRefreshDriver)) {
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index 01832dbd4a..c7a47cb873 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -1998,7 +1998,7 @@ PresShell::Initialize(nscoord aWidth, nscoord aHeight)
     }
   }
 
-  if (root && root->IsXUL()) {
+  if (root && root->IsXULElement()) {
     mozilla::Telemetry::AccumulateTimeDelta(Telemetry::XUL_INITIAL_FRAME_CONSTRUCTION,
                                             timerStart);
   }
@@ -3192,7 +3192,7 @@ PresShell::GoToAnchor(const nsAString& aAnchorName, bool aScroll,
   }
 
   const Element *root = mDocument->GetRootElement();
-  if (root && root->IsSVG(nsGkAtoms::svg)) {
+  if (root && root->IsSVGElement(nsGkAtoms::svg)) {
     // We need to execute this even if there is an empty anchor name
     // so that any existing SVG fragment identifier effect is removed
     if (SVGFragmentIdentifier::ProcessFragmentIdentifier(mDocument, aAnchorName)) {
@@ -3235,7 +3235,7 @@ PresShell::GoToAnchor(const nsAString& aAnchorName, bool aScroll,
         // Ensure it's an anchor element
         content = do_QueryInterface(node);
         if (content) {
-          if (content->Tag() == nsGkAtoms::a && content->IsHTML()) {
+          if (content->IsHTMLElement(nsGkAtoms::a)) {
             break;
           }
           content = nullptr;
@@ -6117,7 +6117,7 @@ PresShell::AssumeAllImagesVisible()
       mPresContext->Type() == nsPresContext::eContext_Print ||
       mPresContext->IsChrome() ||
       mDocument->IsResourceDoc() ||
-      mDocument->IsXUL()) {
+      mDocument->IsXULDocument()) {
     return true;
   }
 
@@ -7103,7 +7103,7 @@ BuildTargetChainForBeforeAfterKeyboardEvent(nsINode* aTarget,
                                             bool& aTargetIsIframe)
 {
   nsCOMPtr<nsIContent> content(do_QueryInterface(aTarget));
-  aTargetIsIframe = content && content->IsHTML(nsGkAtoms::iframe);
+  aTargetIsIframe = content && content->IsHTMLElement(nsGkAtoms::iframe);
 
   Element* frameElement;
   // If event target is not an iframe, skip the event target and get its
@@ -7542,8 +7542,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
                        "Unexpected document");
           nsIFrame* captureFrame = capturingContent->GetPrimaryFrame();
           if (captureFrame) {
-            if (capturingContent->Tag() == nsGkAtoms::select &&
-                capturingContent->IsHTML()) {
+            if (capturingContent->IsHTMLElement(nsGkAtoms::select)) {
               // a dropdown <select> has a child in its selectPopupList and we should
               // capture on that instead.
               nsIFrame* childFrame = captureFrame->GetChildList(nsIFrame::kSelectPopupList).FirstChild();
@@ -9645,7 +9644,7 @@ PresShell::ProcessReflowCommands(bool aInterruptible)
     int32_t intElapsed = int32_t(elapsed.ToMilliseconds());
 
     Telemetry::ID id;
-    if (mDocument->GetRootElement()->IsXUL()) {
+    if (mDocument->GetRootElement()->IsXULElement()) {
       id = mIsActive
         ? Telemetry::XUL_FOREGROUND_REFLOW_MS
         : Telemetry::XUL_BACKGROUND_REFLOW_MS;
diff --git a/layout/forms/nsImageControlFrame.cpp b/layout/forms/nsImageControlFrame.cpp
index 4a473bda24..9774bd6b00 100644
--- a/layout/forms/nsImageControlFrame.cpp
+++ b/layout/forms/nsImageControlFrame.cpp
@@ -111,8 +111,7 @@ NS_QUERYFRAME_TAIL_INHERITING(nsImageControlFrameSuper)
 a11y::AccType
 nsImageControlFrame::AccessibleType()
 {
-  if (mContent->Tag() == nsGkAtoms::button ||
-      mContent->Tag() == nsGkAtoms::input) {
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::button, nsGkAtoms::input)) {
     return a11y::eHTMLButtonType;
   }
 
diff --git a/layout/forms/nsListControlFrame.cpp b/layout/forms/nsListControlFrame.cpp
index ab4e7ed1e3..856836e96b 100644
--- a/layout/forms/nsListControlFrame.cpp
+++ b/layout/forms/nsListControlFrame.cpp
@@ -706,7 +706,7 @@ CountOptionsAndOptgroups(nsIFrame* aFrame)
     nsIFrame* child = e.get();
     nsIContent* content = child->GetContent();
     if (content) {
-      if (content->IsHTML(nsGkAtoms::option)) {
+      if (content->IsHTMLElement(nsGkAtoms::option)) {
         ++count;
       } else {
         nsCOMPtr<nsIDOMHTMLOptGroupElement> optgroup = do_QueryInterface(content);
diff --git a/layout/forms/nsNumberControlFrame.cpp b/layout/forms/nsNumberControlFrame.cpp
index 8a9045693c..4389c088a8 100644
--- a/layout/forms/nsNumberControlFrame.cpp
+++ b/layout/forms/nsNumberControlFrame.cpp
@@ -530,7 +530,7 @@ nsNumberControlFrame::GetNumberControlFrameForTextField(nsIFrame* aFrame)
   if (content->IsInNativeAnonymousSubtree() &&
       content->GetParent() && content->GetParent()->GetParent()) {
     nsIContent* grandparent = content->GetParent()->GetParent();
-    if (grandparent->IsHTML(nsGkAtoms::input) &&
+    if (grandparent->IsHTMLElement(nsGkAtoms::input) &&
         grandparent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                                  nsGkAtoms::number, eCaseMatters)) {
       return do_QueryFrame(grandparent->GetPrimaryFrame());
@@ -552,7 +552,7 @@ nsNumberControlFrame::GetNumberControlFrameForSpinButton(nsIFrame* aFrame)
       content->GetParent() && content->GetParent()->GetParent() &&
       content->GetParent()->GetParent()->GetParent()) {
     nsIContent* greatgrandparent = content->GetParent()->GetParent()->GetParent();
-    if (greatgrandparent->IsHTML(nsGkAtoms::input) &&
+    if (greatgrandparent->IsHTMLElement(nsGkAtoms::input) &&
         greatgrandparent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::type,
                                       nsGkAtoms::number, eCaseMatters)) {
       return do_QueryFrame(greatgrandparent->GetPrimaryFrame());
diff --git a/layout/forms/nsRangeFrame.cpp b/layout/forms/nsRangeFrame.cpp
index fc75d31823..6f254e5592 100644
--- a/layout/forms/nsRangeFrame.cpp
+++ b/layout/forms/nsRangeFrame.cpp
@@ -450,7 +450,7 @@ nsRangeFrame::AccessibleType()
 double
 nsRangeFrame::GetValueAsFractionOfRange()
 {
-  MOZ_ASSERT(mContent->IsHTML(nsGkAtoms::input), "bad cast");
+  MOZ_ASSERT(mContent->IsHTMLElement(nsGkAtoms::input), "bad cast");
   dom::HTMLInputElement* input = static_cast<dom::HTMLInputElement*>(mContent);
 
   MOZ_ASSERT(input->GetType() == NS_FORM_INPUT_RANGE);
@@ -479,7 +479,7 @@ nsRangeFrame::GetValueAtEventPoint(WidgetGUIEvent* aEvent)
              aEvent->mClass == eTouchEventClass,
              "Unexpected event type - aEvent->refPoint may be meaningless");
 
-  MOZ_ASSERT(mContent->IsHTML(nsGkAtoms::input), "bad cast");
+  MOZ_ASSERT(mContent->IsHTMLElement(nsGkAtoms::input), "bad cast");
   dom::HTMLInputElement* input = static_cast<dom::HTMLInputElement*>(mContent);
 
   MOZ_ASSERT(input->GetType() == NS_FORM_INPUT_RANGE);
@@ -710,7 +710,7 @@ nsRangeFrame::AttributeChanged(int32_t  aNameSpaceID,
       // HTMLInputElement. Given that we're changing away from being a range
       // and this frame will shortly be destroyed, there's no point in calling
       // UpdateForValueChange() anyway.
-      MOZ_ASSERT(mContent->IsHTML(nsGkAtoms::input), "bad cast");
+      MOZ_ASSERT(mContent->IsHTMLElement(nsGkAtoms::input), "bad cast");
       bool typeIsRange = static_cast<dom::HTMLInputElement*>(mContent)->GetType() ==
                            NS_FORM_INPUT_RANGE;
       // If script changed the <input>'s type before setting these attributes
diff --git a/layout/forms/nsSelectsAreaFrame.cpp b/layout/forms/nsSelectsAreaFrame.cpp
index 63f27af334..b067751998 100644
--- a/layout/forms/nsSelectsAreaFrame.cpp
+++ b/layout/forms/nsSelectsAreaFrame.cpp
@@ -53,7 +53,7 @@ void nsDisplayOptionEventGrabber::HitTest(nsDisplayListBuilder* aBuilder,
     nsIFrame* selectedFrame = outFrames.ElementAt(i);
     while (selectedFrame &&
            !(selectedFrame->GetContent() &&
-             selectedFrame->GetContent()->IsHTML(nsGkAtoms::option))) {
+             selectedFrame->GetContent()->IsHTMLElement(nsGkAtoms::option))) {
       selectedFrame = selectedFrame->GetParent();
     }
     if (selectedFrame) {
diff --git a/layout/forms/nsTextControlFrame.cpp b/layout/forms/nsTextControlFrame.cpp
index 866acc9316..fbbdcafc78 100644
--- a/layout/forms/nsTextControlFrame.cpp
+++ b/layout/forms/nsTextControlFrame.cpp
@@ -843,7 +843,7 @@ nsTextControlFrame::SelectAllOrCollapseToEndOfText(bool aSelect)
     // br under the root node!
     nsIContent *child = rootContent->GetChildAt(numChildren - 1);
     if (child) {
-      if (child->Tag() == nsGkAtoms::br)
+      if (child->IsHTMLElement(nsGkAtoms::br))
         --numChildren;
     }
     if (!aSelect && numChildren) {
diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index 6d9379e87a..abbd50542c 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -2922,7 +2922,8 @@ nsBlockFrame::AttributeChanged(int32_t         aNameSpaceID,
     return rv;
   }
   if (nsGkAtoms::start == aAttribute ||
-      (nsGkAtoms::reversed == aAttribute && mContent->IsHTML(nsGkAtoms::ol))) {
+      (nsGkAtoms::reversed == aAttribute &&
+       mContent->IsHTMLElement(nsGkAtoms::ol))) {
     nsPresContext* presContext = PresContext();
 
     // XXX Not sure if this is necessary anymore
@@ -6415,8 +6416,7 @@ static void ComputeVisualOverflowArea(nsLineList& aLines,
 bool
 nsBlockFrame::IsVisibleInSelection(nsISelection* aSelection)
 {
-  if (mContent->IsHTML() && (mContent->Tag() == nsGkAtoms::html ||
-                             mContent->Tag() == nsGkAtoms::body))
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::html, nsGkAtoms::body))
     return true;
 
   nsCOMPtr<nsIDOMNode> node(do_QueryInterface(mContent));
@@ -6640,7 +6640,7 @@ nsBlockFrame::AccessibleType()
   }
 
   // block frame may be for <hr>
-  if (mContent->Tag() == nsGkAtoms::hr) {
+  if (mContent->IsHTMLElement(nsGkAtoms::hr)) {
     return a11y::eHTMLHRType;
   }
 
@@ -6964,7 +6964,7 @@ bool
 nsBlockFrame::FrameStartsCounterScope(nsIFrame* aFrame)
 {
   nsIContent* content = aFrame->GetContent();
-  if (!content || !content->IsHTML())
+  if (!content || !content->IsHTMLElement())
     return false;
 
   nsIAtom *localName = content->NodeInfo()->NameAtom();
@@ -6983,14 +6983,14 @@ nsBlockFrame::RenumberLists(nsPresContext* aPresContext)
     return false;
   }
 
-  MOZ_ASSERT(mContent->IsHTML(),
+  MOZ_ASSERT(mContent->IsHTMLElement(),
              "FrameStartsCounterScope should only return true for HTML elements");
 
   // Setup initial list ordinal value
   // XXX Map html's start property to counter-reset style
   int32_t ordinal = 1;
   int32_t increment;
-  if (mContent->Tag() == nsGkAtoms::ol &&
+  if (mContent->IsHTMLElement(nsGkAtoms::ol) &&
       mContent->HasAttr(kNameSpaceID_None, nsGkAtoms::reversed)) {
     increment = -1;
   } else {
@@ -7010,7 +7010,7 @@ nsBlockFrame::RenumberLists(nsPresContext* aPresContext)
     ordinal = 0;
     for (nsIContent* kid = mContent->GetFirstChild(); kid;
          kid = kid->GetNextSibling()) {
-      if (kid->IsHTML(nsGkAtoms::li)) {
+      if (kid->IsHTMLElement(nsGkAtoms::li)) {
         // FIXME: This isn't right in terms of what CSS says to do for
         // overflow of counters (but it only matters when this node has
         // more than numeric_limits<int32_t>::max() children).
diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index b7045ebaab..f739a416c1 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -627,7 +627,7 @@ nsContainerFrame::SyncWindowProperties(nsPresContext*       aPresContext,
     return;
 
   Element* rootElement = aPresContext->Document()->GetRootElement();
-  if (!rootElement || !rootElement->IsXUL()) {
+  if (!rootElement || !rootElement->IsXULElement()) {
     // Scrollframes use native widgets which don't work well with
     // translucent windows, at least in Windows XP. So if the document
     // has a root scrollrame it's useless to try to make it transparent,
diff --git a/layout/generic/nsFontInflationData.cpp b/layout/generic/nsFontInflationData.cpp
index 8093a62e33..d069c97176 100644
--- a/layout/generic/nsFontInflationData.cpp
+++ b/layout/generic/nsFontInflationData.cpp
@@ -292,7 +292,7 @@ DoCharCountOfLargestOption(nsIFrame *aContainer)
   for (nsIFrame* option = aContainer->GetFirstPrincipalChild();
        option; option = option->GetNextSibling()) {
     uint32_t optionResult;
-    if (option->GetContent()->IsHTML(nsGkAtoms::optgroup)) {
+    if (option->GetContent()->IsHTMLElement(nsGkAtoms::optgroup)) {
       optionResult = DoCharCountOfLargestOption(option);
     } else {
       // REVIEW: Check the frame structure for this!
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 878d35d332..b6dc52ebea 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -491,9 +491,9 @@ IsFontSizeInflationContainer(nsIFrame* aFrame,
                    // outer one should be considered a container.
                    // (Important, e.g., for nsSelectsAreaFrame.)
                    (aFrame->GetParent()->GetContent() == content) ||
-                   (content && (content->IsHTML(nsGkAtoms::option) ||
-                                content->IsHTML(nsGkAtoms::optgroup) ||
-                                content->IsHTML(nsGkAtoms::select) ||
+                   (content && (content->IsAnyOfHTMLElements(nsGkAtoms::option,
+                                                             nsGkAtoms::optgroup,
+                                                             nsGkAtoms::select) ||
                                 content->IsInNativeAnonymousSubtree()))) &&
                   !(aFrame->IsBoxFrame() && aFrame->GetParent()->IsBoxFrame());
   NS_ASSERTION(!aFrame->IsFrameOfType(nsIFrame::eLineParticipant) ||
@@ -1648,9 +1648,9 @@ inline static bool IsSVGContentWithCSSClip(const nsIFrame *aFrame)
   // elements regardless of the value of the 'position' property. Here we obey
   // the CSS spec for outer-<svg> (since that's what we generally do), but
   // obey the SVG spec for other SVG elements to which 'clip' applies.
-  nsIAtom *tag = aFrame->GetContent()->Tag();
   return (aFrame->GetStateBits() & NS_FRAME_SVG_LAYOUT) &&
-    (tag == nsGkAtoms::svg || tag == nsGkAtoms::foreignObject);
+          aFrame->GetContent()->IsAnyOfSVGElements(nsGkAtoms::svg,
+                                                   nsGkAtoms::foreignObject);
 }
 
 bool
@@ -5756,7 +5756,7 @@ nsFrame::MakeFrameName(const nsAString& aType, nsAString& aResult) const
   aResult = aType;
   if (mContent && !mContent->IsNodeOfType(nsINode::eTEXT)) {
     nsAutoString buf;
-    mContent->Tag()->ToString(buf);
+    mContent->NodeInfo()->NameAtom()->ToString(buf);
     if (GetType() == nsGkAtoms::subDocumentFrame) {
       nsAutoString src;
       mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::src, src);
@@ -8034,7 +8034,7 @@ nsIFrame::IsFocusable(int32_t *aTabIndex, bool aWithMouse)
     isFocusable = mContent->IsFocusable(&tabIndex, aWithMouse);
     if (!isFocusable && !aWithMouse &&
         GetType() == nsGkAtoms::scrollFrame &&
-        mContent->IsHTML() &&
+        mContent->IsHTMLElement() &&
         !mContent->IsRootOfNativeAnonymousSubtree() &&
         mContent->GetParent() &&
         !mContent->HasAttr(kNameSpaceID_None, nsGkAtoms::tabindex)) {
@@ -8805,7 +8805,7 @@ nsIFrame::CreateOwnLayerIfNeeded(nsDisplayListBuilder* aBuilder,
                                  nsDisplayList* aList)
 {
   if (GetContent() &&
-      GetContent()->IsXUL() &&
+      GetContent()->IsXULElement() &&
       GetContent()->HasAttr(kNameSpaceID_None, nsGkAtoms::layer)) {
     aList->AppendNewToTop(new (aBuilder) 
         nsDisplayOwnLayer(aBuilder, this, aList));
@@ -8960,7 +8960,7 @@ GetTagName(nsFrame* aFrame, nsIContent* aContent, int aResultSize,
 {
   if (aContent) {
     PR_snprintf(aResult, aResultSize, "%s@%p",
-                nsAtomCString(aContent->Tag()).get(), aFrame);
+                nsAtomCString(aContent->NodeInfo()->NameAtom()).get(), aFrame);
   }
   else {
     PR_snprintf(aResult, aResultSize, "@%p", aFrame);
diff --git a/layout/generic/nsFrameSetFrame.cpp b/layout/generic/nsFrameSetFrame.cpp
index e55deb3585..b9d9cd4f92 100644
--- a/layout/generic/nsFrameSetFrame.cpp
+++ b/layout/generic/nsFrameSetFrame.cpp
@@ -342,16 +342,15 @@ nsHTMLFramesetFrame::Init(nsIContent*       aContent,
 
     // IMPORTANT: This must match the conditions in
     // nsCSSFrameConstructor::ContentAppended/Inserted/Removed
-    if (!child->IsHTML())
+    if (!child->IsHTMLElement())
       continue;
 
-    nsIAtom *tag = child->Tag();
-    if (tag == nsGkAtoms::frameset || tag == nsGkAtoms::frame) {
+    if (child->IsAnyOfHTMLElements(nsGkAtoms::frameset, nsGkAtoms::frame)) {
       nsRefPtr<nsStyleContext> kidSC;
 
       kidSC = shell->StyleSet()->ResolveStyleFor(child->AsElement(),
                                                  mStyleContext);
-      if (tag == nsGkAtoms::frameset) {
+      if (child->IsHTMLElement(nsGkAtoms::frameset)) {
         frame = NS_NewHTMLFramesetFrame(shell, kidSC);
 
         nsHTMLFramesetFrame* childFrame = (nsHTMLFramesetFrame*)frame;
diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index dc22016581..acc92b6130 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3773,10 +3773,10 @@ ScrollFrameHelper::ReloadChildFrames()
           NS_ASSERTION(!mVScrollbarBox, "Found multiple vertical scrollbars?");
           mVScrollbarBox = frame;
         }
-      } else if (content->Tag() == nsGkAtoms::resizer) {
+      } else if (content->IsXULElement(nsGkAtoms::resizer)) {
         NS_ASSERTION(!mResizerBox, "Found multiple resizers");
         mResizerBox = frame;
-      } else if (content->Tag() == nsGkAtoms::scrollcorner) {
+      } else if (content->IsXULElement(nsGkAtoms::scrollcorner)) {
         // probably a scrollcorner
         NS_ASSERTION(!mScrollCornerBox, "Found multiple scrollcorners");
         mScrollCornerBox = frame;
diff --git a/layout/generic/nsHTMLReflowState.cpp b/layout/generic/nsHTMLReflowState.cpp
index 178e626448..e7d58708f4 100644
--- a/layout/generic/nsHTMLReflowState.cpp
+++ b/layout/generic/nsHTMLReflowState.cpp
@@ -390,8 +390,8 @@ nsHTMLReflowState::Init(nsPresContext* aPresContext,
               // Don't set NS_FRAME_IN_CONSTRAINED_HEIGHT on body or html
               // elements.
              (frame->GetContent() &&
-            !(frame->GetContent()->IsHTML(nsGkAtoms::body) ||
-              frame->GetContent()->IsHTML(nsGkAtoms::html)))) {
+            !(frame->GetContent()->IsAnyOfHTMLElements(nsGkAtoms::body,
+                                                       nsGkAtoms::html)))) {
 
     // If our height was specified as a percentage, then this could
     // actually resolve to 'auto', based on:
@@ -1794,15 +1794,13 @@ CalcQuirkContainingBlockHeight(const nsHTMLReflowState* aCBReflowState)
       if (firstAncestorRS) {
         nsIContent* frameContent = firstAncestorRS->frame->GetContent();
         if (frameContent) {
-          nsIAtom *contentTag = frameContent->Tag();
-          NS_ASSERTION(contentTag == nsGkAtoms::html, "First ancestor is not HTML");
+          NS_ASSERTION(frameContent->IsHTMLElement(nsGkAtoms::html), "First ancestor is not HTML");
         }
       }
       if (secondAncestorRS) {
         nsIContent* frameContent = secondAncestorRS->frame->GetContent();
         if (frameContent) {
-          nsIAtom *contentTag = frameContent->Tag();
-          NS_ASSERTION(contentTag == nsGkAtoms::body, "Second ancestor is not BODY");
+          NS_ASSERTION(frameContent->IsHTMLElement(nsGkAtoms::body), "Second ancestor is not BODY");
         }
       }
 #endif
diff --git a/layout/generic/nsImageFrame.cpp b/layout/generic/nsImageFrame.cpp
index bc954b12f1..de02655cc4 100644
--- a/layout/generic/nsImageFrame.cpp
+++ b/layout/generic/nsImageFrame.cpp
@@ -477,8 +477,8 @@ nsImageFrame::ShouldCreateImageFrameFor(Element* aElement,
   }
   else if (aElement->HasAttr(kNameSpaceID_None, nsGkAtoms::src) &&
            !aElement->HasAttr(kNameSpaceID_None, nsGkAtoms::alt) &&
-           !aElement->IsHTML(nsGkAtoms::object) &&
-           !aElement->IsHTML(nsGkAtoms::input)) {
+           !aElement->IsHTMLElement(nsGkAtoms::object) &&
+           !aElement->IsHTMLElement(nsGkAtoms::input)) {
     // Use a sized box if we have no alt text.  This means no alt attribute
     // and the node is not an object or an input (since those always have alt
     // text).
@@ -1324,7 +1324,8 @@ nsImageFrame::DisplayAltFeedback(nsRenderingContext& aRenderingContext,
     nsIContent* content = GetContent();
     if (content) {
       nsXPIDLString altText;
-      nsCSSFrameConstructor::GetAlternateTextFor(content, content->Tag(),
+      nsCSSFrameConstructor::GetAlternateTextFor(content,
+                                                 content->NodeInfo()->NameAtom(),
                                                  altText);
       DisplayAltText(PresContext(), aRenderingContext, altText, inner);
     }
diff --git a/layout/generic/nsImageMap.cpp b/layout/generic/nsImageMap.cpp
index c27db5d869..049e259906 100644
--- a/layout/generic/nsImageMap.cpp
+++ b/layout/generic/nsImageMap.cpp
@@ -760,27 +760,26 @@ nsImageMap::SearchForAreas(nsIContent* aParent, bool& aFoundArea,
   for (i = 0; i < n; i++) {
     nsIContent *child = aParent->GetChildAt(i);
 
-    if (child->IsHTML()) {
-      // If we haven't determined that the map element contains an
-      // <a> element yet, then look for <area>.
-      if (!aFoundAnchor && child->Tag() == nsGkAtoms::area) {
-        aFoundArea = true;
-        rv = AddArea(child);
-        NS_ENSURE_SUCCESS(rv, rv);
+    // If we haven't determined that the map element contains an
+    // <a> element yet, then look for <area>.
+    if (!aFoundAnchor && child->IsHTMLElement(nsGkAtoms::area)) {
+      aFoundArea = true;
+      rv = AddArea(child);
+      NS_ENSURE_SUCCESS(rv, rv);
 
-        // Continue to next child. This stops mContainsBlockContents from
-        // getting set. It also makes us ignore children of <area>s which
-        // is consistent with how we react to dynamic insertion of such
-        // children.
-        continue;
-      }
-      // If we haven't determined that the map element contains an
-      // <area> element yet, then look for <a>.
-      if (!aFoundArea && child->Tag() == nsGkAtoms::a) {
-        aFoundAnchor = true;
-        rv = AddArea(child);
-        NS_ENSURE_SUCCESS(rv, rv);
-      }
+      // Continue to next child. This stops mContainsBlockContents from
+      // getting set. It also makes us ignore children of <area>s which
+      // is consistent with how we react to dynamic insertion of such
+      // children.
+      continue;
+    }
+
+    // If we haven't determined that the map element contains an
+    // <area> element yet, then look for <a>.
+    if (!aFoundArea && child->IsHTMLElement(nsGkAtoms::a)) {
+      aFoundAnchor = true;
+      rv = AddArea(child);
+      NS_ENSURE_SUCCESS(rv, rv);
     }
 
     if (child->IsElement()) {
@@ -927,7 +926,7 @@ nsImageMap::AttributeChanged(nsIDocument*  aDocument,
   // are the only cases we care about.
   if ((aElement->NodeInfo()->Equals(nsGkAtoms::area) ||
        aElement->NodeInfo()->Equals(nsGkAtoms::a)) &&
-      aElement->IsHTML() &&
+      aElement->IsHTMLElement() &&
       aNameSpaceID == kNameSpaceID_None &&
       (aAttribute == nsGkAtoms::shape ||
        aAttribute == nsGkAtoms::coords)) {
diff --git a/layout/generic/nsInlineFrame.cpp b/layout/generic/nsInlineFrame.cpp
index 693e2a282e..ad790fc32a 100644
--- a/layout/generic/nsInlineFrame.cpp
+++ b/layout/generic/nsInlineFrame.cpp
@@ -1016,10 +1016,9 @@ nsInlineFrame::AccessibleType()
 {
   // Broken image accessibles are created here, because layout
   // replaces the image or image control frame with an inline frame
-  nsIAtom *tagAtom = mContent->Tag();
-  if (tagAtom == nsGkAtoms::input)  // Broken <input type=image ... />
+  if (mContent->IsHTMLElement(nsGkAtoms::input))  // Broken <input type=image ... />
     return a11y::eHTMLButtonType;
-  if (tagAtom == nsGkAtoms::img)  // Create accessible for broken <img>
+  if (mContent->IsHTMLElement(nsGkAtoms::img))  // Create accessible for broken <img>
     return a11y::eHyperTextType;
 
   return a11y::eNoType;
diff --git a/layout/generic/nsLineLayout.cpp b/layout/generic/nsLineLayout.cpp
index af374f9851..2f48b10fbf 100644
--- a/layout/generic/nsLineLayout.cpp
+++ b/layout/generic/nsLineLayout.cpp
@@ -2262,11 +2262,10 @@ nsLineLayout::VerticalAlignFrames(PerSpanData* psd)
     if (!applyMinLH && isLastLine) {
       nsIContent* blockContent = mRootSpan->mFrame->mFrame->GetContent();
       if (blockContent) {
-        nsIAtom *blockTagAtom = blockContent->Tag();
         // (3) above, if the last line of LI, DT, or DD
-        if (blockTagAtom == nsGkAtoms::li ||
-            blockTagAtom == nsGkAtoms::dt ||
-            blockTagAtom == nsGkAtoms::dd) {
+        if (blockContent->IsAnyOfHTMLElements(nsGkAtoms::li,
+                                              nsGkAtoms::dt,
+                                              nsGkAtoms::dd)) {
           applyMinLH = true;
         }
       }
diff --git a/layout/generic/nsPluginFrame.cpp b/layout/generic/nsPluginFrame.cpp
index 043a297cb1..64be0bf7c6 100644
--- a/layout/generic/nsPluginFrame.cpp
+++ b/layout/generic/nsPluginFrame.cpp
@@ -378,8 +378,8 @@ nsPluginFrame::GetMinISize(nsRenderingContext *aRenderingContext)
   nscoord result = 0;
 
   if (!IsHidden(false)) {
-    nsIAtom *atom = mContent->Tag();
-    if (atom == nsGkAtoms::applet || atom == nsGkAtoms::embed) {
+    if (mContent->IsAnyOfHTMLElements(nsGkAtoms::applet,
+                                      nsGkAtoms::embed)) {
       bool vertical = GetWritingMode().IsVertical();
       result = nsPresContext::CSSPixelsToAppUnits(
         vertical ? EMBED_DEF_HEIGHT : EMBED_DEF_WIDTH);
@@ -440,8 +440,8 @@ nsPluginFrame::GetDesiredSize(nsPresContext* aPresContext,
   aMetrics.Height() = aReflowState.ComputedHeight();
 
   // for EMBED and APPLET, default to 240x200 for compatibility
-  nsIAtom *atom = mContent->Tag();
-  if (atom == nsGkAtoms::applet || atom == nsGkAtoms::embed) {
+  if (mContent->IsAnyOfHTMLElements(nsGkAtoms::applet,
+                                    nsGkAtoms::embed)) {
     if (aMetrics.Width() == NS_UNCONSTRAINEDSIZE) {
       aMetrics.Width() = clamped(nsPresContext::CSSPixelsToAppUnits(EMBED_DEF_WIDTH),
                                aReflowState.ComputedMinWidth(),
@@ -731,7 +731,7 @@ nsPluginFrame::IsHidden(bool aCheckVisibilityStyle) const
   }
 
   // only <embed> tags support the HIDDEN attribute
-  if (mContent->Tag() == nsGkAtoms::embed) {
+  if (mContent->IsHTMLElement(nsGkAtoms::embed)) {
     // Yes, these are really the kooky ways that you could tell 4.x
     // not to hide the <embed> once you'd put the 'hidden' attribute
     // on the tag...
diff --git a/layout/generic/nsSelection.cpp b/layout/generic/nsSelection.cpp
index e751c0632e..278607fc74 100644
--- a/layout/generic/nsSelection.cpp
+++ b/layout/generic/nsSelection.cpp
@@ -758,7 +758,7 @@ nsIAtom *GetTag(nsINode *aNode)
     return nullptr;
   }
   
-  return content->Tag();
+  return content->NodeInfo()->NameAtom();
 }
 
 // Returns the parent
@@ -2226,9 +2226,7 @@ nsFrameSelection::NotifySelectionListeners(SelectionType aType)
 
 static bool IsCell(nsIContent *aContent)
 {
-  return ((aContent->Tag() == nsGkAtoms::td ||
-           aContent->Tag() == nsGkAtoms::th) &&
-          aContent->IsHTML());
+  return aContent->IsAnyOfHTMLElements(nsGkAtoms::td, nsGkAtoms::th);
 }
 
 nsITableCellLayout* 
@@ -2984,8 +2982,7 @@ nsFrameSelection::GetParentTable(nsIContent *aCell) const
 
   for (nsIContent* parent = aCell->GetParent(); parent;
        parent = parent->GetParent()) {
-    if (parent->Tag() == nsGkAtoms::table &&
-        parent->IsHTML()) {
+    if (parent->IsHTMLElement(nsGkAtoms::table)) {
       return parent;
     }
   }
@@ -3119,15 +3116,13 @@ Selection::GetTableSelectionType(nsIDOMRange* aDOMRange,
     return NS_OK;
 
   nsIContent* startContent = static_cast<nsIContent*>(startNode);
-  if (!(startNode->IsElement() && startContent->IsHTML())) {
+  if (!(startNode->IsElement() && startContent->IsHTMLElement())) {
     // Implies a check for being an element; if we ever make this work
     // for non-HTML, need to keep checking for elements.
     return NS_OK;
   }
 
-  nsIAtom *tag = startContent->Tag();
-
-  if (tag == nsGkAtoms::tr)
+  if (startContent->IsHTMLElement(nsGkAtoms::tr))
   {
     *aTableSelectionType = nsISelectionPrivate::TABLESELECTION_CELL;
   }
@@ -3137,11 +3132,9 @@ Selection::GetTableSelectionType(nsIDOMRange* aDOMRange,
     if (!child)
       return NS_ERROR_FAILURE;
 
-    tag = child->Tag();
-
-    if (tag == nsGkAtoms::table)
+    if (child->IsHTMLElement(nsGkAtoms::table))
       *aTableSelectionType = nsISelectionPrivate::TABLESELECTION_TABLE;
-    else if (tag == nsGkAtoms::tr)
+    else if (child->IsHTMLElement(nsGkAtoms::tr))
       *aTableSelectionType = nsISelectionPrivate::TABLESELECTION_ROW;
   }
 
@@ -4967,7 +4960,7 @@ Selection::Collapse(nsINode& aParentNode, uint32_t aOffset, ErrorResult& aRv)
   nsCOMPtr<nsIContent> content = do_QueryInterface(&aParentNode);
   nsCOMPtr<nsIDocument> doc = do_QueryInterface(&aParentNode);
   printf ("Sel. Collapse to %p %s %d\n", &aParentNode,
-          content ? nsAtomCString(content->Tag()).get()
+          content ? nsAtomCString(content->NodeInfo()->NameAtom()).get()
                   : (doc ? "DOCUMENT" : "???"),
           aOffset);
 #endif
@@ -5508,7 +5501,7 @@ Selection::Extend(nsINode& aParentNode, uint32_t aOffset, ErrorResult& aRv)
   }
   nsCOMPtr<nsIContent> content = do_QueryInterface(&aParentNode);
   printf ("Sel. Extend to %p %s %d\n", content.get(),
-          nsAtomCString(content->Tag()).get(), aOffset);
+          nsAtomCString(content->NodeInfo()->NameAtom()).get(), aOffset);
 #endif
   res = mFrameSelection->NotifySelectionListeners(GetType());
   if (NS_FAILED(res)) {
diff --git a/layout/generic/nsSubDocumentFrame.cpp b/layout/generic/nsSubDocumentFrame.cpp
index 4fa5b25020..2055ee0f27 100644
--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -595,7 +595,7 @@ nsSubDocumentFrame::GetIntrinsicISize()
     return 0;  // HTML <frame> has no useful intrinsic width
   }
 
-  if (mContent->IsXUL()) {
+  if (mContent->IsXULElement()) {
     return 0;  // XUL <iframe> and <browser> have no useful intrinsic width
   }
 
@@ -613,7 +613,7 @@ nsSubDocumentFrame::GetIntrinsicBSize()
   // <frame> processing does not use this routine, only <iframe>
   NS_ASSERTION(IsInline(), "Shouldn't have been called");
 
-  if (mContent->IsXUL()) {
+  if (mContent->IsXULElement()) {
     return 0;
   }
 
@@ -864,7 +864,7 @@ nsSubDocumentFrame::AttributeChanged(int32_t aNameSpaceID,
   if (aAttribute == nsGkAtoms::noresize) {
     // Note that we're not doing content type checks, but that's ok -- if
     // they'd fail we will just end up with a null framesetFrame.
-    if (mContent->GetParent()->Tag() == nsGkAtoms::frameset) {
+    if (mContent->GetParent()->IsHTMLElement(nsGkAtoms::frameset)) {
       nsIFrame* parentFrame = GetParent();
 
       if (parentFrame) {
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index 70a1761a86..44a84760bd 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -1966,7 +1966,7 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
     if (mLineContainer->HasAnyStateBits(TEXT_IS_IN_TOKEN_MATHML)) {
       // All MathML tokens except <mtext> use 'math' script.
       if (!(parent && parent->GetContent() &&
-          parent->GetContent()->Tag() == nsGkAtoms::mtext_)) {
+          parent->GetContent()->IsMathMLElement(nsGkAtoms::mtext_))) {
         textFlags |= gfxTextRunFactory::TEXT_USE_MATH_SCRIPT;
       }
       nsIMathMLFrame* mathFrame = do_QueryFrame(parent);
@@ -4956,7 +4956,7 @@ nsTextFrame::GetTextDecorations(
 
     // In quirks mode, if we're on an HTML table element, we're done.
     if (compatMode == eCompatibility_NavQuirks &&
-        f->GetContent()->IsHTML(nsGkAtoms::table)) {
+        f->GetContent()->IsHTMLElement(nsGkAtoms::table)) {
       break;
     }
 
diff --git a/layout/mathml/nsMathMLContainerFrame.cpp b/layout/mathml/nsMathMLContainerFrame.cpp
index 6eef296217..13cac4b4c4 100644
--- a/layout/mathml/nsMathMLContainerFrame.cpp
+++ b/layout/mathml/nsMathMLContainerFrame.cpp
@@ -707,8 +707,7 @@ nsMathMLContainerFrame::ReLayoutChildren(nsIFrame* aParentFrame)
     NS_ASSERTION(content, "dangling frame without a content node");
     if (!content)
       break;
-    if (content->GetNameSpaceID() == kNameSpaceID_MathML &&
-        content->Tag() == nsGkAtoms::math)
+    if (content->IsMathMLElement(nsGkAtoms::math))
       break;
 
     frame = parent;
@@ -1250,7 +1249,7 @@ private:
     GetItalicCorrection(mSize.mBoundingMetrics,
                         leftCorrection, rightCorrection);
     if (!mChildFrame->GetPrevSibling() &&
-        mParentFrame->GetContent()->Tag() == nsGkAtoms::msqrt_) {
+        mParentFrame->GetContent()->IsMathMLElement(nsGkAtoms::msqrt_)) {
       // Remove leading correction in <msqrt> because the sqrt glyph itself is
       // there first.
       if (!mRTL) {
@@ -1343,7 +1342,7 @@ nsMathMLContainerFrame::SetIncrementScriptLevel(int32_t aChildIndex, bool aIncre
   if (!child)
     return;
   nsIContent* content = child->GetContent();
-  if (!content->IsMathML())
+  if (!content->IsMathMLElement())
     return;
   nsMathMLElement* element = static_cast<nsMathMLElement*>(content);
 
@@ -1402,9 +1401,8 @@ AddInterFrameSpacingToSize(nsHTMLReflowMetrics&    aDesiredSize,
   if (MOZ_UNLIKELY(!parentContent)) {
     return 0;
   }
-  nsIAtom *parentTag = parentContent->Tag();
-  if (parentContent->GetNameSpaceID() == kNameSpaceID_MathML && 
-      (parentTag == nsGkAtoms::math || parentTag == nsGkAtoms::mtd_)) {
+  if (parentContent->IsAnyOfMathMLElements(nsGkAtoms::math,
+                                           nsGkAtoms::mtd_)) {
     gap = GetInterFrameSpacingFor(aFrame->StyleFont()->mScriptLevel,
                                   parent, aFrame);
     // add our own italic correction
@@ -1568,14 +1566,14 @@ nsMathMLContainerFrame::ReportParseError(const char16_t* aAttribute,
                                          const char16_t* aValue)
 {
   const char16_t* argv[] = 
-    { aValue, aAttribute, mContent->Tag()->GetUTF16String() };
+    { aValue, aAttribute, mContent->NodeInfo()->NameAtom()->GetUTF16String() };
   return ReportErrorToConsole("AttributeParsingError", argv, 3);
 }
 
 nsresult
 nsMathMLContainerFrame::ReportChildCountError()
 {
-  const char16_t* arg = mContent->Tag()->GetUTF16String();
+  const char16_t* arg = mContent->NodeInfo()->NameAtom()->GetUTF16String();
   return ReportErrorToConsole("ChildCountIncorrect", &arg, 1);
 }
 
@@ -1583,7 +1581,8 @@ nsresult
 nsMathMLContainerFrame::ReportInvalidChildError(nsIAtom* aChildTag)
 {
   const char16_t* argv[] =
-    { aChildTag->GetUTF16String(), mContent->Tag()->GetUTF16String() };
+    { aChildTag->GetUTF16String(),
+      mContent->NodeInfo()->NameAtom()->GetUTF16String() };
   return ReportErrorToConsole("InvalidChild", argv, 2);
 }
 
diff --git a/layout/mathml/nsMathMLFrame.cpp b/layout/mathml/nsMathMLFrame.cpp
index 30221783ce..60dbe9cbf6 100644
--- a/layout/mathml/nsMathMLFrame.cpp
+++ b/layout/mathml/nsMathMLFrame.cpp
@@ -158,7 +158,7 @@ nsMathMLFrame::GetPresentationDataFrom(nsIFrame*           aFrame,
     if (!content)
       break;
 
-    if (content->Tag() == nsGkAtoms::math) {
+    if (content->IsMathMLElement(nsGkAtoms::math)) {
       break;
     }
     frame = frame->GetParent();
diff --git a/layout/mathml/nsMathMLTokenFrame.cpp b/layout/mathml/nsMathMLTokenFrame.cpp
index 56ae4f4dca..98c9a50c9b 100644
--- a/layout/mathml/nsMathMLTokenFrame.cpp
+++ b/layout/mathml/nsMathMLTokenFrame.cpp
@@ -37,7 +37,7 @@ eMathMLFrameType
 nsMathMLTokenFrame::GetMathMLFrameType()
 {
   // treat everything other than <mi> as ordinary...
-  if (mContent->Tag() != nsGkAtoms::mi_) {
+  if (!mContent->IsMathMLElement(nsGkAtoms::mi_)) {
     return eMathMLFrameType_Ordinary;
   }
 
@@ -75,7 +75,7 @@ nsMathMLTokenFrame::MarkTextFramesAsTokenMathML()
       }
     }
   }
-  if (mContent->Tag() == nsGkAtoms::mi_ && childCount == 1) {
+  if (mContent->IsMathMLElement(nsGkAtoms::mi_) && childCount == 1) {
     nsAutoString data;
     if (!nsContentUtils::GetNodeTextContent(mContent, false, data)) {
       NS_RUNTIMEABORT("OOM");
diff --git a/layout/mathml/nsMathMLTokenFrame.h b/layout/mathml/nsMathMLTokenFrame.h
index a98ef6c295..4200fb5b26 100644
--- a/layout/mathml/nsMathMLTokenFrame.h
+++ b/layout/mathml/nsMathMLTokenFrame.h
@@ -23,7 +23,7 @@ public:
   TransmitAutomaticData() override {
     // The REC defines the following elements to be space-like:
     // * an mtext, mspace, maligngroup, or malignmark element;
-    if (mContent->Tag() == nsGkAtoms::mtext_) {
+    if (mContent->IsMathMLElement(nsGkAtoms::mtext_)) {
       mPresentationData.flags |= NS_MATHML_SPACE_LIKE;
     }
     return NS_OK;
diff --git a/layout/mathml/nsMathMLmactionFrame.cpp b/layout/mathml/nsMathMLmactionFrame.cpp
index 5ff699013b..593f020fbf 100644
--- a/layout/mathml/nsMathMLmactionFrame.cpp
+++ b/layout/mathml/nsMathMLmactionFrame.cpp
@@ -291,8 +291,7 @@ nsMathMLmactionFrame::MouseOver()
     if (!content) return;
 
     // check whether the content is mtext or not
-    if (content->GetNameSpaceID() == kNameSpaceID_MathML &&
-        content->Tag() == nsGkAtoms::mtext_) {
+    if (content->IsMathMLElement(nsGkAtoms::mtext_)) {
       // get the text to be displayed
       content = content->GetFirstChild();
       if (!content) return;
diff --git a/layout/mathml/nsMathMLmmultiscriptsFrame.cpp b/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
index b5a85460d3..2ee6212f45 100644
--- a/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
+++ b/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
@@ -37,7 +37,7 @@ nsMathMLmmultiscriptsFrame::ScriptIncrement(nsIFrame* aFrame)
     return 0;
   if (mFrames.ContainsFrame(aFrame)) {
     if (mFrames.FirstChild() == aFrame ||
-        aFrame->GetContent()->Tag() == nsGkAtoms::mprescripts_) {
+        aFrame->GetContent()->IsMathMLElement(nsGkAtoms::mprescripts_)) {
       return 0; // No script increment for base frames or prescript markers
     }
     return 1;
@@ -57,12 +57,12 @@ nsMathMLmmultiscriptsFrame::TransmitAutomaticData()
   // the compression flag in them.
 
   int32_t count = 0;
-  bool isSubScript = mContent->Tag() != nsGkAtoms::msup_;
+  bool isSubScript = !mContent->IsMathMLElement(nsGkAtoms::msup_);
 
   nsAutoTArray<nsIFrame*, 8> subScriptFrames;
   nsIFrame* childFrame = mFrames.FirstChild();
   while (childFrame) {
-    if (childFrame->GetContent()->Tag() == nsGkAtoms::mprescripts_) {
+    if (childFrame->GetContent()->IsMathMLElement(nsGkAtoms::mprescripts_)) {
       // mprescripts frame
     } else if (0 == count) {
       // base frame
@@ -96,7 +96,6 @@ nsMathMLmmultiscriptsFrame::Place(nsRenderingContext& aRenderingContext,
 {
   nscoord subScriptShift = 0;
   nscoord supScriptShift = 0;
-  nsIAtom* tag = mContent->Tag();
   float fontSizeInflation = nsLayoutUtils::FontSizeInflationFor(this);
 
   // subscriptshift
@@ -111,7 +110,7 @@ nsMathMLmmultiscriptsFrame::Place(nsRenderingContext& aRenderingContext,
   // As a minimum, negative values can be ignored.
   //
   nsAutoString value;
-  if (tag != nsGkAtoms::msup_) {
+  if (!mContent->IsMathMLElement(nsGkAtoms::msup_)) {
     mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::subscriptshift_, value);
     if (!value.IsEmpty()) {
       ParseNumericValue(value, &subScriptShift, 0, PresContext(),
@@ -129,7 +128,7 @@ nsMathMLmmultiscriptsFrame::Place(nsRenderingContext& aRenderingContext,
   // We use 0 as the default value so unitless values can be ignored.
   // As a minimum, negative values can be ignored.
   //
-  if (tag != nsGkAtoms::msub_) {
+  if (!mContent->IsMathMLElement(nsGkAtoms::msub_)) {
     mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::superscriptshift_, value);
     if (!value.IsEmpty()) {
       ParseNumericValue(value, &supScriptShift, 0, PresContext(),
@@ -153,16 +152,16 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*      aPresContext,
                                         nscoord              aUserSupScriptShift,
                                         float                aFontSizeInflation)
 {
-  nsIAtom* tag = aFrame->GetContent()->Tag();
+  nsIAtom* tag = aFrame->GetContent()->NodeInfo()->NameAtom();
 
   // This function deals with both munderover etc. as well as msubsup etc.
   // As the former behaves identically to the later, we treat it as such
   // to avoid additional checks later.
-  if (tag == nsGkAtoms::mover_)
+  if (aFrame->GetContent()->IsMathMLElement(nsGkAtoms::mover_))
     tag = nsGkAtoms::msup_;
-  else if (tag == nsGkAtoms::munder_)
+  else if (aFrame->GetContent()->IsMathMLElement(nsGkAtoms::munder_))
     tag = nsGkAtoms::msub_;
-  else if (tag  == nsGkAtoms::munderover_)
+  else if (aFrame->GetContent()->IsMathMLElement(nsGkAtoms::munderover_))
     tag = nsGkAtoms::msubsup_;
 
   nsBoundingMetrics bmFrame;
@@ -325,11 +324,10 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*      aPresContext,
 
   nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
   while (childFrame) {
-    nsIAtom* childTag = childFrame->GetContent()->Tag();
-    if (childTag == nsGkAtoms::mprescripts_) {
+    if (childFrame->GetContent()->IsMathMLElement(nsGkAtoms::mprescripts_)) {
       if (tag != nsGkAtoms::mmultiscripts_) {
         if (aPlaceOrigin) {
-          aFrame->ReportInvalidChildError(childTag);
+          aFrame->ReportInvalidChildError(nsGkAtoms::mprescripts_);
         }
         return aFrame->ReflowError(aRenderingContext, aDesiredSize);
       }
@@ -353,7 +351,7 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*      aPresContext,
     } else if (0 == count) {
       // base
 
-      if (childTag == nsGkAtoms::none) {
+      if (childFrame->GetContent()->IsMathMLElement(nsGkAtoms::none)) {
         if (tag == nsGkAtoms::mmultiscripts_) {
           if (aPlaceOrigin) {
             aFrame->ReportErrorToConsole("NoBase");
@@ -384,7 +382,7 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*      aPresContext,
       boundingMetrics.leftBearing = bmBase.leftBearing; // until overwritten
     } else {
       // super/subscript block
-      if ( childTag == nsGkAtoms::none) {
+      if (childFrame->GetContent()->IsMathMLElement(nsGkAtoms::none)) {
         foundNoneTag = true;
       }
 
diff --git a/layout/mathml/nsMathMLmrowFrame.cpp b/layout/mathml/nsMathMLmrowFrame.cpp
index 493a82abf7..5bdcff1706 100644
--- a/layout/mathml/nsMathMLmrowFrame.cpp
+++ b/layout/mathml/nsMathMLmrowFrame.cpp
@@ -41,7 +41,7 @@ nsMathMLmrowFrame::AttributeChanged(int32_t  aNameSpaceID,
   // Special for <mtable>: In the frame construction code, we also use
   // this frame class as a wrapper for mtable. Hence, we should pass the
   // notification to the real mtable
-  if (mContent->Tag() == nsGkAtoms::mtable_) {
+  if (mContent->IsMathMLElement(nsGkAtoms::mtable_)) {
     nsIFrame* frame = mFrames.FirstChild();
     for ( ; frame; frame = frame->GetFirstPrincipalChild()) {
       // drill down to the real mtable
diff --git a/layout/mathml/nsMathMLmtableFrame.cpp b/layout/mathml/nsMathMLmtableFrame.cpp
index 10ab8f0c3e..8548a55678 100644
--- a/layout/mathml/nsMathMLmtableFrame.cpp
+++ b/layout/mathml/nsMathMLmtableFrame.cpp
@@ -116,7 +116,7 @@ static nsresult ReportParseError(nsIFrame* aFrame, const char16_t* aAttribute,
   nsIContent* content = aFrame->GetContent();
 
   const char16_t* params[] =
-    { aValue, aAttribute, content->Tag()->GetUTF16String() };
+    { aValue, aAttribute, content->NodeInfo()->NameAtom()->GetUTF16String() };
 
   return nsContentUtils::ReportToConsole(nsIScriptError::errorFlag,
                                          NS_LITERAL_CSTRING("Layout: MathML"),
@@ -643,7 +643,8 @@ ListMathMLTree(nsIFrame* atLeast)
   nsIFrame* f = atLeast;
   for ( ; f; f = f->GetParent()) {
     nsIContent* c = f->GetContent();
-    if (!c || c->Tag() == nsGkAtoms::math || c->Tag() == nsGkAtoms::body)
+    if (!c || c->IsMathMLElement(nsGkAtoms::math) ||
+        c->NodeInfo()->NameAtom(nsGkAtoms::body))  // XXXbaku which kind of body tag?
       break;
   }
   if (!f) f = atLeast;
@@ -1135,7 +1136,8 @@ nsMathMLmtdFrame::GetRowSpan()
   int32_t rowspan = 1;
 
   // Don't look at the content's rowspan if we're not an mtd or a pseudo cell.
-  if ((mContent->Tag() == nsGkAtoms::mtd_) && !StyleContext()->GetPseudo()) {
+  if (mContent->IsMathMLElement(nsGkAtoms::mtd_) &&
+      !StyleContext()->GetPseudo()) {
     nsAutoString value;
     mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::rowspan, value);
     if (!value.IsEmpty()) {
@@ -1155,7 +1157,8 @@ nsMathMLmtdFrame::GetColSpan()
   int32_t colspan = 1;
 
   // Don't look at the content's colspan if we're not an mtd or a pseudo cell.
-  if ((mContent->Tag() == nsGkAtoms::mtd_) && !StyleContext()->GetPseudo()) {
+  if (mContent->IsMathMLElement(nsGkAtoms::mtd_) &&
+      !StyleContext()->GetPseudo()) {
     nsAutoString value;
     mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::columnspan_, value);
     if (!value.IsEmpty()) {
diff --git a/layout/mathml/nsMathMLmunderoverFrame.cpp b/layout/mathml/nsMathMLmunderoverFrame.cpp
index 20badfb74c..a465dc9dd4 100644
--- a/layout/mathml/nsMathMLmunderoverFrame.cpp
+++ b/layout/mathml/nsMathMLmunderoverFrame.cpp
@@ -79,7 +79,7 @@ nsMathMLmunderoverFrame::ScriptIncrement(nsIFrame* aFrame)
   }
   child = child->GetNextSibling();
   if (aFrame == child) {
-    if (mContent->Tag() == nsGkAtoms::mover_) {
+    if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
       return mIncrementOver ? 1 : 0;
     }
     return mIncrementUnder ? 1 : 0;
@@ -128,19 +128,19 @@ XXX The winner is the outermost setting in conflicting settings like these:
   nsIFrame* overscriptFrame = nullptr;
   nsIFrame* underscriptFrame = nullptr;
   nsIFrame* baseFrame = mFrames.FirstChild();
-  nsIAtom* tag = mContent->Tag();
 
   if (baseFrame) {
-    if (tag == nsGkAtoms::munder_ ||
-        tag == nsGkAtoms::munderover_) {
+    if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                        nsGkAtoms::munderover_)) {
       underscriptFrame = baseFrame->GetNextSibling();
     } else {
-      NS_ASSERTION(tag == nsGkAtoms::mover_, "mContent->Tag() not recognized");
+      NS_ASSERTION(mContent->IsMathMLElement(nsGkAtoms::mover_),
+                   "mContent->NodeInfo()->NameAtom() not recognized");
       overscriptFrame = baseFrame->GetNextSibling();
     }
   }
   if (underscriptFrame &&
-      tag == nsGkAtoms::munderover_) {
+      mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
     overscriptFrame = underscriptFrame->GetNextSibling();
 
   }
@@ -155,8 +155,8 @@ XXX The winner is the outermost setting in conflicting settings like these:
   // and its core <mo> is an accent
   nsEmbellishData embellishData;
   nsAutoString value;
-  if (tag == nsGkAtoms::munder_ ||
-      tag == nsGkAtoms::munderover_) {
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                      nsGkAtoms::munderover_)) {
     GetEmbellishDataFrom(underscriptFrame, embellishData);
     if (NS_MATHML_EMBELLISH_IS_ACCENT(embellishData.flags)) {
       mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTUNDER;
@@ -176,8 +176,8 @@ XXX The winner is the outermost setting in conflicting settings like these:
 
   // The default value of accent is false, unless the overscript is embellished
   // and its core <mo> is an accent
-  if (tag == nsGkAtoms::mover_ ||
-      tag == nsGkAtoms::munderover_) {
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::mover_,
+                                      nsGkAtoms::munderover_)) {
     GetEmbellishDataFrom(overscriptFrame, embellishData);
     if (NS_MATHML_EMBELLISH_IS_ACCENT(embellishData.flags)) {
       mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTOVER;
@@ -224,14 +224,14 @@ XXX The winner is the outermost setting in conflicting settings like these:
      that math accents and \overline change uncramped styles to their
      cramped counterparts.
   */
-  if (tag == nsGkAtoms::mover_ ||
-      tag == nsGkAtoms::munderover_) {
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::mover_,
+                                      nsGkAtoms::munderover_)) {
     uint32_t compress = NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags)
       ? NS_MATHML_COMPRESSED : 0;
     mIncrementOver =
       !NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags) ||
       subsupDisplay;
-    SetIncrementScriptLevel(tag == nsGkAtoms::mover_ ? 1 : 2, mIncrementOver);
+    SetIncrementScriptLevel(mContent->IsMathMLElement(nsGkAtoms::mover_) ? 1 : 2, mIncrementOver);
     if (mIncrementOver) {
       PropagateFrameFlagFor(overscriptFrame,
                             NS_FRAME_MATHML_SCRIPT_DESCENDANT);
@@ -242,8 +242,8 @@ XXX The winner is the outermost setting in conflicting settings like these:
      The TeXBook treats 'under' like a subscript, so p.141 or Rule 13a 
      say it should be compressed
   */
-  if (tag == nsGkAtoms::munder_ ||
-      tag == nsGkAtoms::munderover_) {
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                      nsGkAtoms::munderover_)) {
     mIncrementUnder =
       !NS_MATHML_EMBELLISH_IS_ACCENTUNDER(mEmbellishData.flags) ||
       subsupDisplay;
@@ -306,18 +306,17 @@ nsMathMLmunderoverFrame::Place(nsRenderingContext& aRenderingContext,
                                nsHTMLReflowMetrics& aDesiredSize)
 {
   float fontSizeInflation = nsLayoutUtils::FontSizeInflationFor(this);
-  nsIAtom* tag = mContent->Tag();
   if (NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishData.flags) &&
       StyleFont()->mMathDisplay == NS_MATHML_DISPLAYSTYLE_INLINE) {
     //place like sub sup or subsup
-    if (tag == nsGkAtoms::munderover_) {
+    if (mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
       return nsMathMLmmultiscriptsFrame::PlaceMultiScript(PresContext(),
                                                           aRenderingContext,
                                                           aPlaceOrigin,
                                                           aDesiredSize,
                                                           this, 0, 0,
                                                           fontSizeInflation);
-    } else if (tag == nsGkAtoms::munder_) {
+    } else if (mContent->IsMathMLElement( nsGkAtoms::munder_)) {
       return nsMathMLmmultiscriptsFrame::PlaceMultiScript(PresContext(),
                                                           aRenderingContext,
                                                           aPlaceOrigin,
@@ -325,7 +324,8 @@ nsMathMLmunderoverFrame::Place(nsRenderingContext& aRenderingContext,
                                                           this, 0, 0,
                                                           fontSizeInflation);
     } else {
-      NS_ASSERTION(tag == nsGkAtoms::mover_, "mContent->Tag() not recognized");
+      NS_ASSERTION(mContent->IsMathMLElement(nsGkAtoms::mover_),
+                   "mContent->NodeInfo()->NameAtom() not recognized");
       return nsMathMLmmultiscriptsFrame::PlaceMultiScript(PresContext(),
                                                           aRenderingContext,
                                                           aPlaceOrigin,
@@ -350,30 +350,30 @@ nsMathMLmunderoverFrame::Place(nsRenderingContext& aRenderingContext,
   overSize.SetBlockStartAscent(0);
   bool haveError = false;
   if (baseFrame) {
-    if (tag == nsGkAtoms::munder_ ||
-        tag == nsGkAtoms::munderover_) {
+    if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                        nsGkAtoms::munderover_)) {
       underFrame = baseFrame->GetNextSibling();
-    } else if (tag == nsGkAtoms::mover_) {
+    } else if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
       overFrame = baseFrame->GetNextSibling();
     }
   }
-  if (underFrame && tag == nsGkAtoms::munderover_) {
+  if (underFrame && mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
     overFrame = underFrame->GetNextSibling();
   }
   
-  if (tag == nsGkAtoms::munder_) {
+  if (mContent->IsMathMLElement(nsGkAtoms::munder_)) {
     if (!baseFrame || !underFrame || underFrame->GetNextSibling()) {
       // report an error, encourage people to get their markups in order
       haveError = true;
     }
   }
-  if (tag == nsGkAtoms::mover_) {
+  if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
     if (!baseFrame || !overFrame || overFrame->GetNextSibling()) {
       // report an error, encourage people to get their markups in order
       haveError = true;
     }
   }
-  if (tag == nsGkAtoms::munderover_) {
+  if (mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
     if (!baseFrame || !underFrame || !overFrame || overFrame->GetNextSibling()) {
       // report an error, encourage people to get their markups in order
       haveError = true;
diff --git a/layout/mathml/nsMathMLsemanticsFrame.cpp b/layout/mathml/nsMathMLsemanticsFrame.cpp
index 0923b98c1b..59029e209b 100644
--- a/layout/mathml/nsMathMLsemanticsFrame.cpp
+++ b/layout/mathml/nsMathMLsemanticsFrame.cpp
@@ -42,9 +42,8 @@ nsMathMLsemanticsFrame::GetSelectedFrame()
   // However some people use this syntax so we take care of this case too.
   bool firstChildIsAnnotation = false;
   nsIContent* childContent = childFrame->GetContent();
-  if (childContent->GetNameSpaceID() == kNameSpaceID_MathML &&
-      (childContent->Tag() == nsGkAtoms::annotation_ ||
-       childContent->Tag() == nsGkAtoms::annotation_xml_)) {
+  if (childContent->IsAnyOfMathMLElements(nsGkAtoms::annotation_,
+                                          nsGkAtoms::annotation_xml_)) {
     firstChildIsAnnotation = true;
   }
 
@@ -67,9 +66,7 @@ nsMathMLsemanticsFrame::GetSelectedFrame()
   for ( ; childFrame; childFrame = childFrame->GetNextSibling()) {
     nsIContent* childContent = childFrame->GetContent();
 
-    if (childContent->GetNameSpaceID() != kNameSpaceID_MathML) continue;
-
-    if (childContent->Tag() == nsGkAtoms::annotation_) {
+    if (childContent->IsMathMLElement(nsGkAtoms::annotation_)) {
 
       // If the <annotation> element has an src attribute we ignore it.
       // XXXfredw Should annotation images be supported? See the related
@@ -82,7 +79,7 @@ nsMathMLsemanticsFrame::GetSelectedFrame()
       break;
     }
 
-    if (childContent->Tag() == nsGkAtoms::annotation_xml_) {
+    if (childContent->IsMathMLElement(nsGkAtoms::annotation_xml_)) {
 
       // If the <annotation-xml> element has an src attribute we ignore it.
       if (childContent->HasAttr(kNameSpaceID_None, nsGkAtoms::src)) continue;
diff --git a/layout/printing/nsPrintEngine.cpp b/layout/printing/nsPrintEngine.cpp
index 686be8f7e0..efaeedf97b 100644
--- a/layout/printing/nsPrintEngine.cpp
+++ b/layout/printing/nsPrintEngine.cpp
@@ -3041,7 +3041,7 @@ bool nsPrintEngine::HasFramesetChild(nsIContent* aContent)
   for (nsIContent* child = aContent->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->IsHTML(nsGkAtoms::frameset)) {
+    if (child->IsHTMLElement(nsGkAtoms::frameset)) {
       return true;
     }
   }
diff --git a/layout/printing/nsPrintPreviewListener.cpp b/layout/printing/nsPrintPreviewListener.cpp
index a0b7c5467c..5a4e7dae45 100644
--- a/layout/printing/nsPrintPreviewListener.cpp
+++ b/layout/printing/nsPrintPreviewListener.cpp
@@ -162,7 +162,7 @@ nsPrintPreviewListener::HandleEvent(nsIDOMEvent* aEvent)
 {
   nsCOMPtr<nsIContent> content = do_QueryInterface(
     aEvent ? aEvent->InternalDOMEvent()->GetOriginalTarget() : nullptr);
-  if (content && !content->IsXUL()) {
+  if (content && !content->IsXULElement()) {
     eEventAction action = ::GetActionForEvent(aEvent);
     switch (action) {
       case eEventAction_Tab:
diff --git a/layout/style/AnimationCommon.cpp b/layout/style/AnimationCommon.cpp
index ddb2bbdba3..983d2196bc 100644
--- a/layout/style/AnimationCommon.cpp
+++ b/layout/style/AnimationCommon.cpp
@@ -662,7 +662,7 @@ AnimationPlayerCollection::LogAsyncAnimationFailure(nsCString& aMessage,
 {
   if (aContent) {
     aMessage.AppendLiteral(" [");
-    aMessage.Append(nsAtomCString(aContent->Tag()));
+    aMessage.Append(nsAtomCString(aContent->NodeInfo()->NameAtom()));
 
     nsIAtom* id = aContent->GetID();
     if (id) {
diff --git a/layout/style/SVGAttrAnimationRuleProcessor.cpp b/layout/style/SVGAttrAnimationRuleProcessor.cpp
index c1323e2a74..3a1230f511 100644
--- a/layout/style/SVGAttrAnimationRuleProcessor.cpp
+++ b/layout/style/SVGAttrAnimationRuleProcessor.cpp
@@ -37,7 +37,7 @@ void
 SVGAttrAnimationRuleProcessor::ElementRulesMatching(Element* aElement,
                                                     nsRuleWalker* aRuleWalker)
 {
-  if (aElement->IsSVG()) {
+  if (aElement->IsSVGElement()) {
     static_cast<nsSVGElement*>(aElement)->
       WalkAnimatedContentStyleRules(aRuleWalker);
   }
diff --git a/layout/style/nsCSSRuleProcessor.cpp b/layout/style/nsCSSRuleProcessor.cpp
index e41a369f5d..8904d9fa55 100644
--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -668,7 +668,7 @@ void RuleHash::EnumerateAllRules(Element* aElement, ElementDependentRuleProcesso
                                  NodeMatchContext& aNodeContext)
 {
   int32_t nameSpace = aElement->GetNameSpaceID();
-  nsIAtom* tag = aElement->Tag();
+  nsIAtom* tag = aElement->NodeInfo()->NameAtom();
   nsIAtom* id = aElement->GetID();
   const nsAttrValue* classList = aElement->GetClasses();
 
@@ -1678,7 +1678,7 @@ StateSelectorMatches(Element* aElement,
                                            NS_EVENT_STATE_HOVER) &&
       aTreeMatchContext.mCompatMode == eCompatibility_NavQuirks &&
       ActiveHoverQuirkMatches(aSelector, aSelectorFlags) &&
-      aElement->IsHTML() && !nsCSSRuleProcessor::IsLink(aElement)) {
+      aElement->IsHTMLElement() && !nsCSSRuleProcessor::IsLink(aElement)) {
     // In quirks mode, only make links sensitive to selectors ":active"
     // and ":hover".
     return false;
@@ -1757,9 +1757,9 @@ static bool SelectorMatches(Element* aElement,
 
   if (aSelector->mLowercaseTag) {
     nsIAtom* selectorTag =
-      (aTreeMatchContext.mIsHTMLDocument && aElement->IsHTML()) ?
+      (aTreeMatchContext.mIsHTMLDocument && aElement->IsHTMLElement()) ?
         aSelector->mLowercaseTag : aSelector->mCasedTag;
-    if (selectorTag != aElement->Tag()) {
+    if (selectorTag != aElement->NodeInfo()->NameAtom()) {
       return false;
     }
   }
@@ -1870,7 +1870,7 @@ static bool SelectorMatches(Element* aElement,
           } while (child &&
                    (!IsSignificantChild(child, true, false) ||
                     (child->GetNameSpaceID() == aElement->GetNameSpaceID() &&
-                     child->Tag()->Equals(nsDependentString(pseudoClass->u.mString)))));
+                     child->NodeInfo()->NameAtom()->Equals(nsDependentString(pseudoClass->u.mString)))));
           if (child != nullptr) {
             return false;
           }
@@ -2073,7 +2073,7 @@ static bool SelectorMatches(Element* aElement,
         break;
 
       case nsCSSPseudoClasses::ePseudoClass_mozIsHTML:
-        if (!aTreeMatchContext.mIsHTMLDocument || !aElement->IsHTML()) {
+        if (!aTreeMatchContext.mIsHTMLDocument || !aElement->IsHTMLElement()) {
           return false;
         }
         break;
@@ -2152,7 +2152,7 @@ static bool SelectorMatches(Element* aElement,
 
       case nsCSSPseudoClasses::ePseudoClass_mozTableBorderNonzero:
         {
-          if (!aElement->IsHTML(nsGkAtoms::table)) {
+          if (!aElement->IsHTMLElement(nsGkAtoms::table)) {
             return false;
           }
           const nsAttrValue *val = aElement->GetParsedAttr(nsGkAtoms::border);
@@ -2248,7 +2248,7 @@ static bool SelectorMatches(Element* aElement,
 
       do {
         bool isHTML =
-          (aTreeMatchContext.mIsHTMLDocument && aElement->IsHTML());
+          (aTreeMatchContext.mIsHTMLDocument && aElement->IsHTMLElement());
         matchAttribute = isHTML ? attr->mLowercaseAttr : attr->mCasedAttr;
         if (attr->mNameSpace == kNameSpaceID_Unknown) {
           // Attr selector with a wildcard namespace.  We have to examine all
@@ -3709,7 +3709,7 @@ AncestorFilter::PushAncestor(Element *aElement)
 #ifdef DEBUG
   mElements.AppendElement(aElement);
 #endif
-  mHashes.AppendElement(aElement->Tag()->hash());
+  mHashes.AppendElement(aElement->NodeInfo()->NameAtom()->hash());
   nsIAtom *id = aElement->GetID();
   if (id) {
     mHashes.AppendElement(id->hash());
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 48ba0b6c6c..be0e83117c 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -434,11 +434,11 @@ nsComputedDOMStyle::GetStyleContextForElementNoFlush(Element* aElement,
       return nullptr;
   }
 
-  // XXX the !aElement->IsHTML(nsGkAtoms::area)
+  // XXX the !aElement->IsHTMLElement(nsGkAtoms::area)
   // check is needed due to bug 135040 (to avoid using 
   // mPrimaryFrame). Remove it once that's fixed.
   if (!aPseudo && aStyleType == eAll && inDocWithShell &&
-      !aElement->IsHTML(nsGkAtoms::area)) {
+      !aElement->IsHTMLElement(nsGkAtoms::area)) {
     nsIFrame* frame = nsLayoutUtils::GetStyleFrame(aElement);
     if (frame) {
       nsStyleContext* result = frame->StyleContext();
@@ -606,10 +606,11 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
     return;
   }
 
-  // XXX the !mContent->IsHTML(nsGkAtoms::area)
+  // XXX the !mContent->IsHTMLElement(nsGkAtoms::area)
   // check is needed due to bug 135040 (to avoid using 
   // mPrimaryFrame). Remove it once that's fixed.
-  if (!mPseudo && mStyleType == eAll && !mContent->IsHTML(nsGkAtoms::area)) {
+  if (!mPseudo && mStyleType == eAll &&
+      !mContent->IsHTMLElement(nsGkAtoms::area)) {
     mOuterFrame = mContent->GetPrimaryFrame();
     mInnerFrame = mOuterFrame;
     if (mOuterFrame) {
diff --git a/layout/style/nsHTMLStyleSheet.cpp b/layout/style/nsHTMLStyleSheet.cpp
index 913e07a8be..f39a2160a7 100644
--- a/layout/style/nsHTMLStyleSheet.cpp
+++ b/layout/style/nsHTMLStyleSheet.cpp
@@ -253,11 +253,9 @@ NS_IMPL_ISUPPORTS(nsHTMLStyleSheet, nsIStyleRuleProcessor)
 nsHTMLStyleSheet::RulesMatching(ElementRuleProcessorData* aData)
 {
   nsRuleWalker *ruleWalker = aData->mRuleWalker;
-  if (aData->mElement->IsHTML() && !ruleWalker->AuthorStyleDisabled()) {
-    nsIAtom* tag = aData->mElement->Tag();
-
+  if (!ruleWalker->AuthorStyleDisabled()) {
     // if we have anchor colors, check if this is an anchor with an href
-    if (tag == nsGkAtoms::a) {
+    if (aData->mElement->IsHTMLElement(nsGkAtoms::a)) {
       if (mLinkRule || mVisitedRule || mActiveRule) {
         EventStates state =
           nsCSSRuleProcessor::GetContentStateForVisitedHandling(
@@ -284,10 +282,10 @@ nsHTMLStyleSheet::RulesMatching(ElementRuleProcessorData* aData)
       } // end link/visited/active rules
     } // end A tag
     // add the rule to handle text-align for a <th>
-    else if (tag == nsGkAtoms::th) {
+    else if (aData->mElement->IsHTMLElement(nsGkAtoms::th)) {
       ruleWalker->Forward(mTableTHRule);
     }
-    else if (tag == nsGkAtoms::table) {
+    else if (aData->mElement->IsHTMLElement(nsGkAtoms::table)) {
       if (aData->mTreeMatchContext.mCompatMode == eCompatibility_NavQuirks) {
         ruleWalker->Forward(mTableQuirkColorRule);
       }
@@ -297,7 +295,7 @@ nsHTMLStyleSheet::RulesMatching(ElementRuleProcessorData* aData)
   // just get the style rules from the content.  For SVG we do this even if
   // author style is disabled, because SVG presentational hints aren't
   // considered style.
-  if (!ruleWalker->AuthorStyleDisabled() || aData->mElement->IsSVG()) {
+  if (!ruleWalker->AuthorStyleDisabled() || aData->mElement->IsSVGElement()) {
     aData->mElement->WalkContentStyleRules(ruleWalker);
   }
 
@@ -314,7 +312,7 @@ nsHTMLStyleSheet::RulesMatching(ElementRuleProcessorData* aData)
 /* virtual */ nsRestyleHint
 nsHTMLStyleSheet::HasStateDependentStyle(StateRuleProcessorData* aData)
 {
-  if (aData->mElement->IsHTML(nsGkAtoms::a) &&
+  if (aData->mElement->IsHTMLElement(nsGkAtoms::a) &&
       nsCSSRuleProcessor::IsLink(aData->mElement) &&
       ((mActiveRule && aData->mStateMask.HasState(NS_EVENT_STATE_ACTIVE)) ||
        (mLinkRule && aData->mStateMask.HasState(NS_EVENT_STATE_VISITED)) ||
@@ -353,7 +351,7 @@ nsHTMLStyleSheet::HasAttributeDependentStyle(AttributeRuleProcessorData* aData)
   Element *element = aData->mElement;
   if (aData->mAttribute == nsGkAtoms::href &&
       (mLinkRule || mVisitedRule || mActiveRule) &&
-      element->IsHTML(nsGkAtoms::a)) {
+      element->IsHTMLElement(nsGkAtoms::a)) {
     return eRestyle_Self;
   }
 
@@ -365,7 +363,7 @@ nsHTMLStyleSheet::HasAttributeDependentStyle(AttributeRuleProcessorData* aData)
     // cellpadding on tables is special and requires reresolving all
     // the cells in the table
     if (aData->mAttribute == nsGkAtoms::cellpadding &&
-        element->IsHTML(nsGkAtoms::table)) {
+        element->IsHTMLElement(nsGkAtoms::table)) {
       return eRestyle_Subtree;
     }
     return eRestyle_Self;
diff --git a/layout/style/nsRuleProcessorData.h b/layout/style/nsRuleProcessorData.h
index 1e68c0f28f..60da99188c 100644
--- a/layout/style/nsRuleProcessorData.h
+++ b/layout/style/nsRuleProcessorData.h
@@ -397,7 +397,7 @@ struct MOZ_STACK_CLASS TreeMatchContext {
     , mVisitedHandling(aVisitedHandling)
     , mDocument(aDocument)
     , mScopedRoot(nullptr)
-    , mIsHTMLDocument(aDocument->IsHTML())
+    , mIsHTMLDocument(aDocument->IsHTMLDocument())
     , mCompatMode(aDocument->GetCompatibilityMode())
     , mUsingPrivateBrowsing(false)
     , mSkippingParentDisplayBasedStyleFixup(false)
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 95b1e720d2..8f3d505f96 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -914,7 +914,8 @@ nsStyleSet::GetContext(nsStyleContext* aParentContext,
     }
   }
 
-  if (aElementForAnimation && aElementForAnimation->IsHTML(nsGkAtoms::body) &&
+  if (aElementForAnimation &&
+      aElementForAnimation->IsHTMLElement(nsGkAtoms::body) &&
       aPseudoType == nsCSSPseudoElements::ePseudo_NotPseudoElement &&
       PresContext()->CompatibilityMode() == eCompatibility_NavQuirks) {
     nsIDocument* doc = aElementForAnimation->GetCurrentDoc();
@@ -1682,7 +1683,7 @@ void
 nsStyleSet::WalkDisableTextZoomRule(Element* aElement, nsRuleWalker* aRuleWalker)
 {
   aRuleWalker->SetLevel(eAgentSheet, false, false);
-  if (aElement->IsSVG(nsGkAtoms::text))
+  if (aElement->IsSVGElement(nsGkAtoms::text))
     aRuleWalker->Forward(mDisableTextZoomStyleRule);
 }
 
diff --git a/layout/style/nsStyleUtil.cpp b/layout/style/nsStyleUtil.cpp
index 90329bfa77..7be207edfa 100644
--- a/layout/style/nsStyleUtil.cpp
+++ b/layout/style/nsStyleUtil.cpp
@@ -662,7 +662,7 @@ nsStyleUtil::CSPAllowsInlineStyle(nsIContent* aContent,
     *aRv = NS_OK;
   }
 
-  MOZ_ASSERT(!aContent || aContent->Tag() == nsGkAtoms::style,
+  MOZ_ASSERT(!aContent || aContent->NodeInfo()->NameAtom() == nsGkAtoms::style,
       "aContent passed to CSPAllowsInlineStyle "
       "for an element that is not <style>");
 
diff --git a/layout/style/nsTransitionManager.cpp b/layout/style/nsTransitionManager.cpp
index c4bc322516..dd14863856 100644
--- a/layout/style/nsTransitionManager.cpp
+++ b/layout/style/nsTransitionManager.cpp
@@ -162,9 +162,9 @@ nsTransitionManager::StyleContextChanged(dom::Element *aElement,
     }
 
     NS_ASSERTION((pseudoType == nsCSSPseudoElements::ePseudo_before &&
-                  aElement->IsGeneratedContentContainerForBefore()) ||
+                  aElement->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentbefore) ||
                  (pseudoType == nsCSSPseudoElements::ePseudo_after &&
-                  aElement->IsGeneratedContentContainerForAfter()),
+                  aElement->NodeInfo()->NameAtom() == nsGkAtoms::mozgeneratedcontentafter),
                  "Unexpected aElement coming through");
 
     // Else the element we want to use from now on is the element the
diff --git a/layout/svg/SVGFEImageFrame.cpp b/layout/svg/SVGFEImageFrame.cpp
index 4a214a2ba4..375042707f 100644
--- a/layout/svg/SVGFEImageFrame.cpp
+++ b/layout/svg/SVGFEImageFrame.cpp
@@ -94,7 +94,7 @@ SVGFEImageFrame::Init(nsIContent*       aContent,
                       nsContainerFrame* aParent,
                       nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::feImage),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::feImage),
                "Trying to construct an SVGFEImageFrame for a "
                "content element that doesn't support the right interfaces");
 
diff --git a/layout/svg/SVGTextFrame.cpp b/layout/svg/SVGTextFrame.cpp
index dd3905d6c0..bc5d579b5f 100644
--- a/layout/svg/SVGTextFrame.cpp
+++ b/layout/svg/SVGTextFrame.cpp
@@ -210,7 +210,7 @@ TrimOffsets(uint32_t& aStart, uint32_t& aLength,
 static nsIContent*
 GetFirstNonAAncestor(nsIContent* aContent)
 {
-  while (aContent && aContent->IsSVG(nsGkAtoms::a)) {
+  while (aContent && aContent->IsSVGElement(nsGkAtoms::a)) {
     aContent = aContent->GetParent();
   }
   return aContent;
@@ -240,23 +240,19 @@ GetFirstNonAAncestor(nsIContent* aContent)
 static bool
 IsTextContentElement(nsIContent* aContent)
 {
-  if (!aContent->IsSVG()) {
-    return false;
-  }
-
-  if (aContent->Tag() == nsGkAtoms::text) {
+  if (aContent->IsSVGElement(nsGkAtoms::text)) {
     nsIContent* parent = GetFirstNonAAncestor(aContent->GetParent());
     return !parent || !IsTextContentElement(parent);
   }
 
-  if (aContent->Tag() == nsGkAtoms::textPath) {
+  if (aContent->IsSVGElement(nsGkAtoms::textPath)) {
     nsIContent* parent = GetFirstNonAAncestor(aContent->GetParent());
-    return parent && parent->IsSVG(nsGkAtoms::text);
+    return parent && parent->IsSVGElement(nsGkAtoms::text);
   }
 
-  if (aContent->Tag() == nsGkAtoms::a ||
-      aContent->Tag() == nsGkAtoms::tspan ||
-      aContent->Tag() == nsGkAtoms::altGlyph) {
+  if (aContent->IsAnyOfSVGElements(nsGkAtoms::a,
+                                   nsGkAtoms::tspan,
+                                   nsGkAtoms::altGlyph)) {
     return true;
   }
 
@@ -1726,7 +1722,7 @@ TextFrameIterator::Next()
       if (next) {
         // Descend into this frame, and accumulate its position.
         mCurrentPosition += next->GetPosition();
-        if (next->GetContent()->Tag() == nsGkAtoms::textPath) {
+        if (next->GetContent()->IsSVGElement(nsGkAtoms::textPath)) {
           // Record this <textPath> frame.
           mTextPathFrames.AppendElement(next);
         }
@@ -1747,7 +1743,7 @@ TextFrameIterator::Next()
           }
           // Remove the current frame's position.
           mCurrentPosition -= mCurrentFrame->GetPosition();
-          if (mCurrentFrame->GetContent()->Tag() == nsGkAtoms::textPath) {
+          if (mCurrentFrame->GetContent()->IsSVGElement(nsGkAtoms::textPath)) {
             // Pop off the <textPath> frame if this is a <textPath>.
             mTextPathFrames.TruncateLength(mTextPathFrames.Length() - 1);
           }
@@ -1761,7 +1757,7 @@ TextFrameIterator::Next()
           if (next) {
             // Moving to the next sibling.
             mCurrentPosition += next->GetPosition();
-            if (next->GetContent()->Tag() == nsGkAtoms::textPath) {
+            if (next->GetContent()->IsSVGElement(nsGkAtoms::textPath)) {
               // Record this <textPath> frame.
               mTextPathFrames.AppendElement(next);
             }
@@ -2978,7 +2974,7 @@ SVGTextDrawPathCallbacks::StrokeGeometry()
       GeneralPattern strokePattern;
       nsSVGUtils::MakeStrokePatternFor(mFrame, gfx, &strokePattern, /*aContextPaint*/ nullptr);
       if (strokePattern.GetPattern()) {
-        if (!mFrame->GetParent()->GetContent()->IsSVG()) {
+        if (!mFrame->GetParent()->GetContent()->IsSVGElement()) {
           // The cast that follows would be unsafe
           MOZ_ASSERT(false, "Our nsTextFrame's parent's content should be SVG");
           return;
@@ -3208,7 +3204,7 @@ SVGTextFrame::Init(nsIContent*       aContent,
                    nsContainerFrame* aParent,
                    nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::text), "Content is not an SVG text");
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::text), "Content is not an SVG text");
 
   SVGTextFrameBase::Init(aContent, aParent, aPrevInFlow);
   AddStateBits((aParent->GetStateBits() & NS_STATE_SVG_CLIPPATH_CHILD) |
@@ -3416,7 +3412,7 @@ SVGTextFrame::MutationObserver::AttributeChanged(
                                                 nsIAtom* aAttribute,
                                                 int32_t aModType)
 {
-  if (!aElement->IsSVG()) {
+  if (!aElement->IsSVGElement()) {
     return;
   }
 
@@ -3434,7 +3430,7 @@ SVGTextFrame::HandleAttributeChangeInDescendant(Element* aElement,
                                                 int32_t aNameSpaceID,
                                                 nsIAtom* aAttribute)
 {
-  if (aElement->Tag() == nsGkAtoms::textPath) {
+  if (aElement->IsSVGElement(nsGkAtoms::textPath)) {
     if (aNameSpaceID == kNameSpaceID_None &&
         aAttribute == nsGkAtoms::startOffset) {
       NotifyGlyphMetricsChange();
@@ -4431,7 +4427,7 @@ SVGTextFrame::ResolvePositionsForNode(nsIContent* aContent,
     return true;
   }
 
-  if (aContent->Tag() == nsGkAtoms::textPath) {
+  if (aContent->IsSVGElement(nsGkAtoms::textPath)) {
     // <textPath> elements are as if they are specified with x="0" y="0", but
     // only if they actually have some text content.
     if (HasTextContent(aContent)) {
@@ -4443,7 +4439,7 @@ SVGTextFrame::ResolvePositionsForNode(nsIContent* aContent,
       mPositions[aIndex].mPosition = gfxPoint();
       mPositions[aIndex].mStartOfChunk = true;
     }
-  } else if (aContent->Tag() != nsGkAtoms::a) {
+  } else if (!aContent->IsSVGElement(nsGkAtoms::a)) {
     // We have a text content element that can have x/y/dx/dy/rotate attributes.
     nsSVGElement* element = static_cast<nsSVGElement*>(aContent);
 
@@ -4541,7 +4537,7 @@ SVGTextFrame::ResolvePositionsForNode(nsIContent* aContent,
   }
 
   // Recurse to children.
-  bool inTextPath = aInTextPath || aContent->Tag() == nsGkAtoms::textPath;
+  bool inTextPath = aInTextPath || aContent->IsSVGElement(nsGkAtoms::textPath);
   for (nsIContent* child = aContent->GetFirstChild();
        child;
        child = child->GetNextSibling()) {
@@ -4552,7 +4548,7 @@ SVGTextFrame::ResolvePositionsForNode(nsIContent* aContent,
     }
   }
 
-  if (aContent->Tag() == nsGkAtoms::textPath) {
+  if (aContent->IsSVGElement(nsGkAtoms::textPath)) {
     // Force a new anchored chunk just after a <textPath>.
     aForceStartOfChunk = true;
   }
@@ -4840,7 +4836,7 @@ SVGTextFrame::GetTextPathPathElement(nsIFrame* aTextPathFrame)
   }
 
   Element* element = property->GetReferencedElement();
-  return (element && element->IsSVG(nsGkAtoms::path)) ?
+  return (element && element->IsSVGElement(nsGkAtoms::path)) ?
     static_cast<SVGPathElement*>(element) : nullptr;
 }
 
diff --git a/layout/svg/SVGViewFrame.cpp b/layout/svg/SVGViewFrame.cpp
index d1c99b4010..256f6ed9eb 100644
--- a/layout/svg/SVGViewFrame.cpp
+++ b/layout/svg/SVGViewFrame.cpp
@@ -83,7 +83,7 @@ SVGViewFrame::Init(nsIContent*       aContent,
                    nsContainerFrame* aParent,
                    nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::view),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::view),
                "Content is not an SVG view");
 
   SVGViewFrameBase::Init(aContent, aParent, aPrevInFlow);
@@ -109,7 +109,7 @@ SVGViewFrame::AttributeChanged(int32_t  aNameSpaceID,
        aAttribute == nsGkAtoms::viewTarget)) {
 
     nsSVGOuterSVGFrame *outerSVGFrame = nsSVGUtils::GetOuterSVGFrame(this);
-    NS_ASSERTION(outerSVGFrame->GetContent()->IsSVG(nsGkAtoms::svg),
+    NS_ASSERTION(outerSVGFrame->GetContent()->IsSVGElement(nsGkAtoms::svg),
                  "Expecting an <svg> element");
 
     SVGSVGElement* svgElement =
diff --git a/layout/svg/nsFilterInstance.cpp b/layout/svg/nsFilterInstance.cpp
index 49b91f8fd8..43f66a5f6d 100644
--- a/layout/svg/nsFilterInstance.cpp
+++ b/layout/svg/nsFilterInstance.cpp
@@ -49,7 +49,7 @@ nsFilterInstance::GetFilterDescription(nsIContent* aFilteredElement,
 static UniquePtr<UserSpaceMetrics>
 UserSpaceMetricsForFrame(nsIFrame* aFrame)
 {
-  if (aFrame->GetContent()->IsSVG()) {
+  if (aFrame->GetContent()->IsSVGElement()) {
     nsSVGElement* element = static_cast<nsSVGElement*>(aFrame->GetContent());
     return MakeUnique<SVGElementMetrics>(element);
   }
diff --git a/layout/svg/nsSVGAFrame.cpp b/layout/svg/nsSVGAFrame.cpp
index e3a193b31b..a7ad04c72d 100644
--- a/layout/svg/nsSVGAFrame.cpp
+++ b/layout/svg/nsSVGAFrame.cpp
@@ -79,7 +79,7 @@ nsSVGAFrame::Init(nsIContent*       aContent,
                   nsContainerFrame* aParent,
                   nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::a),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::a),
                "Trying to construct an SVGAFrame for a "
                "content element that doesn't support the right interfaces");
 
diff --git a/layout/svg/nsSVGClipPathFrame.cpp b/layout/svg/nsSVGClipPathFrame.cpp
index ca2c9fc436..418a87e291 100644
--- a/layout/svg/nsSVGClipPathFrame.cpp
+++ b/layout/svg/nsSVGClipPathFrame.cpp
@@ -128,7 +128,7 @@ nsSVGClipPathFrame::ApplyClipOrPaintClipMask(gfxContext& aContext,
       gfxMatrix toChildsUserSpace = mMatrixForChildren;
       nsIFrame* child = do_QueryFrame(SVGFrame);
       nsIContent* childContent = child->GetContent();
-      if (childContent->IsSVG()) {
+      if (childContent->IsSVGElement()) {
         toChildsUserSpace =
           static_cast<const nsSVGElement*>(childContent)->
             PrependLocalTransformsTo(mMatrixForChildren,
@@ -328,7 +328,7 @@ nsSVGClipPathFrame::Init(nsIContent*       aContent,
                          nsContainerFrame* aParent,
                          nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::clipPath),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::clipPath),
                "Content is not an SVG clipPath!");
 
   AddStateBits(NS_STATE_SVG_CLIPPATH_CHILD);
diff --git a/layout/svg/nsSVGContainerFrame.cpp b/layout/svg/nsSVGContainerFrame.cpp
index 7d8eb5579b..bb93e80832 100644
--- a/layout/svg/nsSVGContainerFrame.cpp
+++ b/layout/svg/nsSVGContainerFrame.cpp
@@ -144,7 +144,7 @@ nsSVGDisplayContainerFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                              const nsDisplayListSet& aLists)
 {
   // mContent could be a XUL element so check for an SVG element before casting
-  if (mContent->IsSVG() &&
+  if (mContent->IsSVGElement() &&
       !static_cast<const nsSVGElement*>(mContent)->HasValidDimensions()) {
     return;
   }
@@ -228,7 +228,7 @@ nsSVGDisplayContainerFrame::IsSVGTransformed(gfx::Matrix *aOwnTransform,
   }
 
   // mContent could be a XUL element so check for an SVG element before casting
-  if (mContent->IsSVG()) {
+  if (mContent->IsSVGElement()) {
     nsSVGElement *content = static_cast<nsSVGElement*>(mContent);
     nsSVGAnimatedTransformList* transformList =
       content->GetAnimatedTransformList();
@@ -264,7 +264,7 @@ nsSVGDisplayContainerFrame::PaintSVG(gfxContext& aContext,
   }
 
   gfxMatrix matrix = aTransform;
-  if (GetContent()->IsSVG()) { // must check before cast
+  if (GetContent()->IsSVGElement()) { // must check before cast
     matrix = static_cast<const nsSVGElement*>(GetContent())->
                PrependLocalTransformsTo(matrix,
                                         nsSVGElement::eChildToUserSpace);
@@ -279,7 +279,7 @@ nsSVGDisplayContainerFrame::PaintSVG(gfxContext& aContext,
     // PaintFrameWithEffects() expects the transform that is passed to it to
     // include the transform to the passed frame's user space, so add it:
     const nsIContent* content = kid->GetContent();
-    if (content->IsSVG()) { // must check before cast
+    if (content->IsSVGElement()) { // must check before cast
       const nsSVGElement* element = static_cast<const nsSVGElement*>(content);
       if (!element->HasValidDimensions()) {
         continue; // nothing to paint for kid
@@ -367,7 +367,7 @@ nsSVGDisplayContainerFrame::ReflowSVG()
                    "expected kid to be a NS_FRAME_IS_NONDISPLAY frame");
       if (kid->GetStateBits() & NS_FRAME_IS_DIRTY) {
         nsSVGContainerFrame* container = do_QueryFrame(kid);
-        if (container && container->GetContent()->IsSVG()) {
+        if (container && container->GetContent()->IsSVGElement()) {
           ReflowSVGNonDisplayText(container);
         }
       }
@@ -381,8 +381,8 @@ nsSVGDisplayContainerFrame::ReflowSVG()
   // come from transforms, which are accounted for by nsDisplayTransform.
   // Note that we rely on |overflow:visible| to allow display list items to be
   // created for our children.
-  MOZ_ASSERT(mContent->Tag() == nsGkAtoms::svg ||
-             (mContent->Tag() == nsGkAtoms::use &&
+  MOZ_ASSERT(mContent->IsSVGElement(nsGkAtoms::svg) ||
+             (mContent->IsSVGElement(nsGkAtoms::use) &&
               mRect.Size() == nsSize(0,0)) ||
              mRect.IsEqualEdges(nsRect()),
              "Only inner-<svg>/<use> is expected to have mRect set");
@@ -423,11 +423,11 @@ nsSVGDisplayContainerFrame::GetBBoxContribution(
     nsIContent *content = kid->GetContent();
     nsISVGChildFrame* svgKid = do_QueryFrame(kid);
     // content could be a XUL element so check for an SVG element before casting
-    if (svgKid && (!content->IsSVG() ||
+    if (svgKid && (!content->IsSVGElement() ||
                    static_cast<const nsSVGElement*>(content)->HasValidDimensions())) {
 
       gfxMatrix transform = gfx::ThebesMatrix(aToBBoxUserspace);
-      if (content->IsSVG()) {
+      if (content->IsSVGElement()) {
         transform = static_cast<nsSVGElement*>(content)->
                       PrependLocalTransformsTo(transform);
       }
diff --git a/layout/svg/nsSVGEffects.cpp b/layout/svg/nsSVGEffects.cpp
index 4eae6c1bd9..a4049ece02 100644
--- a/layout/svg/nsSVGEffects.cpp
+++ b/layout/svg/nsSVGEffects.cpp
@@ -383,7 +383,7 @@ bool
 nsSVGTextPathProperty::TargetIsValid()
 {
   Element* target = GetTarget();
-  return target && target->IsSVG(nsGkAtoms::path);
+  return target && target->IsSVGElement(nsGkAtoms::path);
 }
 
 void
diff --git a/layout/svg/nsSVGFilterFrame.cpp b/layout/svg/nsSVGFilterFrame.cpp
index 74ac8c7ada..f21d0e137f 100644
--- a/layout/svg/nsSVGFilterFrame.cpp
+++ b/layout/svg/nsSVGFilterFrame.cpp
@@ -189,7 +189,7 @@ nsSVGFilterFrame::Init(nsIContent*       aContent,
                        nsContainerFrame* aParent,
                        nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::filter),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::filter),
                "Content is not an SVG filter");
 
   nsSVGFilterFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGForeignObjectFrame.cpp b/layout/svg/nsSVGForeignObjectFrame.cpp
index 734875b9c8..3c398a5024 100644
--- a/layout/svg/nsSVGForeignObjectFrame.cpp
+++ b/layout/svg/nsSVGForeignObjectFrame.cpp
@@ -56,7 +56,7 @@ nsSVGForeignObjectFrame::Init(nsIContent*       aContent,
                               nsContainerFrame* aParent,
                               nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::foreignObject),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::foreignObject),
                "Content is not an SVG foreignObject!");
 
   nsSVGForeignObjectFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGGFrame.cpp b/layout/svg/nsSVGGFrame.cpp
index 0c23a7d2c5..76daf71f95 100644
--- a/layout/svg/nsSVGGFrame.cpp
+++ b/layout/svg/nsSVGGFrame.cpp
@@ -33,7 +33,7 @@ nsSVGGFrame::Init(nsIContent*       aContent,
                   nsContainerFrame* aParent,
                   nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG() &&
+  NS_ASSERTION(aContent->IsSVGElement() &&
                static_cast<nsSVGElement*>(aContent)->IsTransformable(),
                "The element doesn't support nsIDOMSVGTransformable");
 
diff --git a/layout/svg/nsSVGGradientFrame.cpp b/layout/svg/nsSVGGradientFrame.cpp
index 51cfe827c8..528acaeaec 100644
--- a/layout/svg/nsSVGGradientFrame.cpp
+++ b/layout/svg/nsSVGGradientFrame.cpp
@@ -198,7 +198,7 @@ static void GetStopInformation(nsIFrame* aStopFrame,
                                float *aStopOpacity)
 {
   nsIContent* stopContent = aStopFrame->GetContent();
-  MOZ_ASSERT(stopContent && stopContent->IsSVG(nsGkAtoms::stop));
+  MOZ_ASSERT(stopContent && stopContent->IsSVGElement(nsGkAtoms::stop));
 
   static_cast<SVGStopElement*>(stopContent)->
     GetAnimatedNumberValues(aOffset, nullptr);
@@ -410,7 +410,7 @@ nsSVGLinearGradientFrame::Init(nsIContent*       aContent,
                                nsContainerFrame* aParent,
                                nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::linearGradient),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::linearGradient),
                "Content is not an SVG linearGradient");
 
   nsSVGLinearGradientFrameBase::Init(aContent, aParent, aPrevInFlow);
@@ -519,7 +519,7 @@ nsSVGRadialGradientFrame::Init(nsIContent*       aContent,
                                nsContainerFrame* aParent,
                                nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::radialGradient),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::radialGradient),
                "Content is not an SVG radialGradient");
 
   nsSVGRadialGradientFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGImageFrame.cpp b/layout/svg/nsSVGImageFrame.cpp
index 0030130dba..1071f9c904 100644
--- a/layout/svg/nsSVGImageFrame.cpp
+++ b/layout/svg/nsSVGImageFrame.cpp
@@ -142,7 +142,7 @@ nsSVGImageFrame::Init(nsIContent*       aContent,
                       nsContainerFrame* aParent,
                       nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::image),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::image),
                "Content is not an SVG image!");
 
   nsSVGImageFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGInnerSVGFrame.cpp b/layout/svg/nsSVGInnerSVGFrame.cpp
index 9be678fcda..035fcaddd3 100644
--- a/layout/svg/nsSVGInnerSVGFrame.cpp
+++ b/layout/svg/nsSVGInnerSVGFrame.cpp
@@ -41,7 +41,7 @@ nsSVGInnerSVGFrame::Init(nsIContent*       aContent,
                          nsContainerFrame* aParent,
                          nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::svg),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::svg),
                "Content is not an SVG 'svg' element!");
 
   nsSVGInnerSVGFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGIntegrationUtils.cpp b/layout/svg/nsSVGIntegrationUtils.cpp
index c34af80ad4..c58832620f 100644
--- a/layout/svg/nsSVGIntegrationUtils.cpp
+++ b/layout/svg/nsSVGIntegrationUtils.cpp
@@ -437,7 +437,7 @@ nsSVGIntegrationUtils::PaintFramesWithEffects(gfxContext& aContext,
   bool hasSVGLayout = (aFrame->GetStateBits() & NS_FRAME_SVG_LAYOUT);
   if (hasSVGLayout) {
     nsISVGChildFrame *svgChildFrame = do_QueryFrame(aFrame);
-    if (!svgChildFrame || !aFrame->GetContent()->IsSVG()) {
+    if (!svgChildFrame || !aFrame->GetContent()->IsSVGElement()) {
       NS_ASSERTION(false, "why?");
       return;
     }
diff --git a/layout/svg/nsSVGMarkerFrame.cpp b/layout/svg/nsSVGMarkerFrame.cpp
index 3f5fe80d40..e5471c644c 100644
--- a/layout/svg/nsSVGMarkerFrame.cpp
+++ b/layout/svg/nsSVGMarkerFrame.cpp
@@ -54,7 +54,7 @@ nsSVGMarkerFrame::Init(nsIContent*       aContent,
                        nsContainerFrame* aParent,
                        nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::marker), "Content is not an SVG marker");
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::marker), "Content is not an SVG marker");
 
   nsSVGMarkerFrameBase::Init(aContent, aParent, aPrevInFlow);
 }
diff --git a/layout/svg/nsSVGMaskFrame.cpp b/layout/svg/nsSVGMaskFrame.cpp
index fd0b25ec0b..e03de86d23 100644
--- a/layout/svg/nsSVGMaskFrame.cpp
+++ b/layout/svg/nsSVGMaskFrame.cpp
@@ -273,7 +273,7 @@ nsSVGMaskFrame::GetMaskForMaskedFrame(gfxContext* aContext,
       SVGFrame->NotifySVGChanged(nsISVGChildFrame::TRANSFORM_CHANGED);
     }
     gfxMatrix m = mMatrixForChildren;
-    if (kid->GetContent()->IsSVG()) {
+    if (kid->GetContent()->IsSVGElement()) {
       m = static_cast<nsSVGElement*>(kid->GetContent())->
             PrependLocalTransformsTo(m);
     }
@@ -361,7 +361,7 @@ nsSVGMaskFrame::Init(nsIContent*       aContent,
                      nsContainerFrame* aParent,
                      nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::mask),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::mask),
                "Content is not an SVG mask");
 
   nsSVGMaskFrameBase::Init(aContent, aParent, aPrevInFlow);
diff --git a/layout/svg/nsSVGOuterSVGFrame.cpp b/layout/svg/nsSVGOuterSVGFrame.cpp
index f8736a3c29..a32b9d128d 100644
--- a/layout/svg/nsSVGOuterSVGFrame.cpp
+++ b/layout/svg/nsSVGOuterSVGFrame.cpp
@@ -93,7 +93,7 @@ nsSVGOuterSVGFrame::Init(nsIContent*       aContent,
                          nsContainerFrame* aParent,
                          nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::svg),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::svg),
                "Content is not an SVG 'svg' element!");
 
   AddStateBits(NS_STATE_IS_OUTER_SVG |
diff --git a/layout/svg/nsSVGPathGeometryFrame.cpp b/layout/svg/nsSVGPathGeometryFrame.cpp
index a115e910c4..95c206dd4f 100644
--- a/layout/svg/nsSVGPathGeometryFrame.cpp
+++ b/layout/svg/nsSVGPathGeometryFrame.cpp
@@ -168,7 +168,7 @@ nsSVGPathGeometryFrame::DidSetStyleContext(nsStyleContext* aOldStyleContext)
     if (aOldStyleContext->PeekStyleSVG()) {
       if ((StyleSVG()->mStrokeLinecap !=
              aOldStyleContext->PeekStyleSVG()->mStrokeLinecap) &&
-          element->Tag() == nsGkAtoms::path) {
+          element->IsSVGElement(nsGkAtoms::path)) {
         // If the stroke-linecap changes to or from "butt" then our element
         // needs to update its cached Moz2D Path, since SVGPathData::BuildPath
         // decides whether or not to insert little lines into the path for zero
diff --git a/layout/svg/nsSVGPatternFrame.cpp b/layout/svg/nsSVGPatternFrame.cpp
index 98ee550961..4a18fb1a3b 100644
--- a/layout/svg/nsSVGPatternFrame.cpp
+++ b/layout/svg/nsSVGPatternFrame.cpp
@@ -105,7 +105,7 @@ nsSVGPatternFrame::Init(nsIContent*       aContent,
                         nsContainerFrame* aParent,
                         nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::pattern), "Content is not an SVG pattern");
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::pattern), "Content is not an SVG pattern");
 
   nsSVGPatternFrameBase::Init(aContent, aParent, aPrevInFlow);
 }
@@ -406,7 +406,7 @@ nsSVGPatternFrame::PaintPattern(const DrawTarget* aDrawTarget,
         SVGFrame->NotifySVGChanged(nsISVGChildFrame::TRANSFORM_CHANGED);
       }
       gfxMatrix tm = *(patternWithChildren->mCTM);
-      if (kid->GetContent()->IsSVG()) {
+      if (kid->GetContent()->IsSVGElement()) {
         tm = static_cast<nsSVGElement*>(kid->GetContent())->
               PrependLocalTransformsTo(tm, nsSVGElement::eUserSpaceToParent);
       }
@@ -647,7 +647,7 @@ nsSVGPatternFrame::ConstructCTM(const nsSVGViewBox& aViewBox,
   if (IncludeBBoxScale(aViewBox, aPatternContentUnits, aPatternUnits)) {
     tCTM.Scale(callerBBox.Width(), callerBBox.Height());
   } else {
-    if (targetContent->IsSVG()) {
+    if (targetContent->IsSVGElement()) {
       ctx = static_cast<nsSVGElement*>(targetContent)->GetCtx();
     }
     float scale = MaxExpansion(callerCTM);
@@ -664,7 +664,7 @@ nsSVGPatternFrame::ConstructCTM(const nsSVGViewBox& aViewBox,
   }
 
   float viewportWidth, viewportHeight;
-  if (targetContent->IsSVG()) {
+  if (targetContent->IsSVGElement()) {
     // If we're dealing with an SVG target only retrieve the context once.
     // Calling the nsIFrame* variant of GetAnimValue would look it up on
     // every call.
diff --git a/layout/svg/nsSVGStopFrame.cpp b/layout/svg/nsSVGStopFrame.cpp
index 1f650d221a..7997160420 100644
--- a/layout/svg/nsSVGStopFrame.cpp
+++ b/layout/svg/nsSVGStopFrame.cpp
@@ -79,7 +79,7 @@ nsSVGStopFrame::Init(nsIContent*       aContent,
                      nsContainerFrame* aParent,
                      nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::stop), "Content is not a stop element");
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::stop), "Content is not a stop element");
 
   nsSVGStopFrameBase::Init(aContent, aParent, aPrevInFlow);
 }
diff --git a/layout/svg/nsSVGSwitchFrame.cpp b/layout/svg/nsSVGSwitchFrame.cpp
index 833ef38a3a..22309291e9 100644
--- a/layout/svg/nsSVGSwitchFrame.cpp
+++ b/layout/svg/nsSVGSwitchFrame.cpp
@@ -80,7 +80,7 @@ nsSVGSwitchFrame::Init(nsIContent*       aContent,
                        nsContainerFrame* aParent,
                        nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::svgSwitch),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::svgSwitch),
                "Content is not an SVG switch");
 
   nsSVGSwitchFrameBase::Init(aContent, aParent, aPrevInFlow);
@@ -120,7 +120,7 @@ nsSVGSwitchFrame::PaintSVG(gfxContext& aContext,
   nsIFrame *kid = GetActiveChildFrame();
   if (kid) {
     gfxMatrix tm = aTransform;
-    if (kid->GetContent()->IsSVG()) {
+    if (kid->GetContent()->IsSVGElement()) {
       tm = static_cast<nsSVGElement*>(kid->GetContent())->
              PrependLocalTransformsTo(tm, nsSVGElement::eUserSpaceToParent);
     }
@@ -241,7 +241,7 @@ nsSVGSwitchFrame::GetBBoxContribution(const Matrix &aToBBoxUserspace,
   if (svgKid) {
     nsIContent *content = kid->GetContent();
     gfxMatrix transform = ThebesMatrix(aToBBoxUserspace);
-    if (content->IsSVG()) {
+    if (content->IsSVGElement()) {
       transform = static_cast<nsSVGElement*>(content)->
                     PrependLocalTransformsTo(transform);
     }
diff --git a/layout/svg/nsSVGUseFrame.cpp b/layout/svg/nsSVGUseFrame.cpp
index ad1d54a205..ca4b32005d 100644
--- a/layout/svg/nsSVGUseFrame.cpp
+++ b/layout/svg/nsSVGUseFrame.cpp
@@ -103,7 +103,7 @@ nsSVGUseFrame::Init(nsIContent*       aContent,
                     nsContainerFrame* aParent,
                     nsIFrame*         aPrevInFlow)
 {
-  NS_ASSERTION(aContent->IsSVG(nsGkAtoms::use),
+  NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::use),
                "Content is not an SVG use!");
 
   mHasValidDimensions =
diff --git a/layout/svg/nsSVGUtils.cpp b/layout/svg/nsSVGUtils.cpp
index f207faf774..73efb21f55 100644
--- a/layout/svg/nsSVGUtils.cpp
+++ b/layout/svg/nsSVGUtils.cpp
@@ -490,7 +490,7 @@ nsSVGUtils::PaintFrameWithEffects(nsIFrame *aFrame,
     return;
 
   const nsIContent* content = aFrame->GetContent();
-  if (content->IsSVG() &&
+  if (content->IsSVGElement() &&
       !static_cast<const nsSVGElement*>(content)->HasValidDimensions()) {
     return;
   }
@@ -706,7 +706,7 @@ nsSVGUtils::HitTestChildren(nsSVGDisplayContainerFrame* aFrame,
   // First we transform aPoint into the coordinate space established by aFrame
   // for its children (e.g. take account of any 'viewBox' attribute):
   gfxPoint point = aPoint;
-  if (aFrame->GetContent()->IsSVG()) { // must check before cast
+  if (aFrame->GetContent()->IsSVGElement()) { // must check before cast
     gfxMatrix m = static_cast<const nsSVGElement*>(aFrame->GetContent())->
                     PrependLocalTransformsTo(gfxMatrix(),
                                              nsSVGElement::eChildToUserSpace);
@@ -727,14 +727,14 @@ nsSVGUtils::HitTestChildren(nsSVGDisplayContainerFrame* aFrame,
     nsISVGChildFrame* SVGFrame = do_QueryFrame(current);
     if (SVGFrame) {
       const nsIContent* content = current->GetContent();
-      if (content->IsSVG() &&
+      if (content->IsSVGElement() &&
           !static_cast<const nsSVGElement*>(content)->HasValidDimensions()) {
         continue;
       }
       // GetFrameForPoint() expects a point in its frame's SVG user space, so
       // we need to convert to that space:
       gfxPoint p = point;
-      if (content->IsSVG()) { // must check before cast
+      if (content->IsSVGElement()) { // must check before cast
         gfxMatrix m = static_cast<const nsSVGElement*>(content)->
                         PrependLocalTransformsTo(gfxMatrix(),
                                                  nsSVGElement::eUserSpaceToParent);
@@ -899,7 +899,7 @@ nsSVGUtils::GetBBox(nsIFrame *aFrame, uint32_t aFlags)
       svg = do_QueryFrame(ancestor);
     }
     nsIContent* content = aFrame->GetContent();
-    if (content->IsSVG() &&
+    if (content->IsSVGElement() &&
         !static_cast<const nsSVGElement*>(content)->HasValidDimensions()) {
       return bbox;
     }
@@ -911,7 +911,7 @@ nsSVGUtils::GetBBox(nsIFrame *aFrame, uint32_t aFlags)
       // needs investigation to check that we won't break too much content.
       // NOTE: When changing this to apply to other frame types, make sure to
       // also update nsSVGUtils::FrameSpaceInCSSPxToUserSpaceOffset.
-      MOZ_ASSERT(content->IsSVG(), "bad cast");
+      MOZ_ASSERT(content->IsSVGElement(), "bad cast");
       nsSVGElement *element = static_cast<nsSVGElement*>(content);
       matrix = element->PrependLocalTransformsTo(matrix,
                           nsSVGElement::eChildToUserSpace);
@@ -1037,7 +1037,7 @@ nsSVGUtils::GetRelativeRect(uint16_t aUnits, const nsSVGLength2 *aXYWH,
     return GetBoundingBoxRelativeRect(aXYWH, aBBox);
   }
   nsIContent* content = aFrame->GetContent();
-  if (content->IsSVG()) {
+  if (content->IsSVGElement()) {
     nsSVGElement* svgElement = static_cast<nsSVGElement*>(content);
     return GetRelativeRect(aUnits, aXYWH, aBBox, SVGElementMetrics(svgElement));
   }
@@ -1113,7 +1113,7 @@ nsSVGUtils::GetNonScalingStrokeTransform(nsIFrame *aFrame,
   }
 
   nsIContent *content = aFrame->GetContent();
-  MOZ_ASSERT(content->IsSVG(), "bad cast");
+  MOZ_ASSERT(content->IsSVGElement(), "bad cast");
 
   *aUserToOuterSVG = ThebesMatrix(SVGContentUtils::GetCTM(
                        static_cast<nsSVGElement*>(content), true));
@@ -1161,10 +1161,9 @@ nsSVGUtils::PathExtentsToMaxStrokeExtents(const gfxRect& aPathExtents,
                                           nsSVGPathGeometryFrame* aFrame,
                                           const gfxMatrix& aMatrix)
 {
-  const nsIAtom* tag = aFrame->GetContent()->Tag();
-
-  bool strokeMayHaveCorners = (tag != nsGkAtoms::circle &&
-                               tag != nsGkAtoms::ellipse);
+  bool strokeMayHaveCorners =
+    !aFrame->GetContent()->IsAnyOfSVGElements(nsGkAtoms::circle,
+                                              nsGkAtoms::ellipse);
 
   // For a shape without corners the stroke can only extend half the stroke
   // width from the path in the x/y-axis directions. For shapes with corners
@@ -1174,9 +1173,11 @@ nsSVGUtils::PathExtentsToMaxStrokeExtents(const gfxRect& aPathExtents,
 
   // The stroke can extend even further for paths that can be affected by
   // stroke-miterlimit.
-  bool affectedByMiterlimit = (tag == nsGkAtoms::path ||
-                               tag == nsGkAtoms::polyline ||
-                               tag == nsGkAtoms::polygon);
+  bool affectedByMiterlimit =
+    aFrame->GetContent()->IsAnyOfSVGElements(nsGkAtoms::path,
+                                             nsGkAtoms::polyline,
+                                             nsGkAtoms::polygon);
+
   if (affectedByMiterlimit) {
     const nsStyleSVG* style = aFrame->StyleSVG();
     if (style->mStrokeLinejoin == NS_STYLE_STROKE_LINEJOIN_MITER &&
@@ -1440,7 +1441,7 @@ GetStrokeDashData(nsIFrame* aFrame,
 
     gfxFloat pathScale = 1.0;
 
-    if (content->Tag() == nsGkAtoms::path) {
+    if (content->IsSVGElement(nsGkAtoms::path)) {
       pathScale = static_cast<SVGPathElement*>(content)->
         GetPathLengthScale(SVGPathElement::eForStroking);
       if (pathScale <= 0) {
@@ -1594,7 +1595,7 @@ nsSVGUtils::PaintSVGGlyph(Element* aElement, gfxContext* aContext,
   aContext->GetDrawTarget()->AddUserData(&gfxTextContextPaint::sUserDataKey,
                                          aContextPaint, nullptr);
   gfxMatrix m;
-  if (frame->GetContent()->IsSVG()) {
+  if (frame->GetContent()->IsSVGElement()) {
     // PaintSVG() expects the passed transform to be the transform to its own
     // SVG user space, so we need to account for any 'transform' attribute:
     m = static_cast<nsSVGElement*>(frame->GetContent())->
@@ -1617,7 +1618,7 @@ nsSVGUtils::GetSVGGlyphExtents(Element* aElement,
 
   gfxMatrix transform(aSVGToAppSpace);
   nsIContent* content = frame->GetContent();
-  if (content->IsSVG()) {
+  if (content->IsSVGElement()) {
     transform = static_cast<nsSVGElement*>(content)->
                   PrependLocalTransformsTo(aSVGToAppSpace);
   }
diff --git a/layout/xul/ListBoxObject.cpp b/layout/xul/ListBoxObject.cpp
index 67be900aad..0235200ce3 100644
--- a/layout/xul/ListBoxObject.cpp
+++ b/layout/xul/ListBoxObject.cpp
@@ -145,7 +145,7 @@ ListBoxObject::GetIndexOfItem(Element& aElement)
 static nsIContent*
 FindBodyContent(nsIContent* aParent)
 {
-  if (aParent->Tag() == nsGkAtoms::listboxbody) {
+  if (aParent->IsXULElement(nsGkAtoms::listboxbody)) {
     return aParent;
   }
 
diff --git a/layout/xul/nsBox.cpp b/layout/xul/nsBox.cpp
index 38c8b9706c..471c3507b4 100644
--- a/layout/xul/nsBox.cpp
+++ b/layout/xul/nsBox.cpp
@@ -468,7 +468,7 @@ nsIFrame::GetOrdinal()
 
   // When present, attribute value overrides CSS.
   nsIContent* content = GetContent();
-  if (content && content->IsXUL()) {
+  if (content && content->IsXULElement()) {
     nsresult error;
     nsAutoString value;
 
@@ -640,7 +640,7 @@ nsIFrame::AddCSSPrefSize(nsIFrame* aBox, nsSize& aSize, bool &aWidthSet, bool &a
     // ignore 'height' and 'width' attributes if the actual element is not XUL
     // For example, we might be magic XUL frames whose primary content is an HTML
     // <select>
-    if (content && content->IsXUL()) {
+    if (content && content->IsXULElement()) {
         nsAutoString value;
         nsresult error;
 
@@ -741,7 +741,7 @@ nsIFrame::AddCSSMinSize(nsBoxLayoutState& aState, nsIFrame* aBox, nsSize& aSize,
     // calc() with percentage is treated like '0' (unset)
 
     nsIContent* content = aBox->GetContent();
-    if (content && content->IsXUL()) {
+    if (content && content->IsXULElement()) {
         nsAutoString value;
         nsresult error;
 
@@ -804,7 +804,7 @@ nsIFrame::AddCSSMaxSize(nsIFrame* aBox, nsSize& aSize, bool &aWidthSet, bool &aH
     // percentages and calc() with percentages are treated like 'none'
 
     nsIContent* content = aBox->GetContent();
-    if (content && content->IsXUL()) {
+    if (content && content->IsXULElement()) {
         nsAutoString value;
         nsresult error;
 
@@ -843,7 +843,7 @@ nsIFrame::AddCSSFlex(nsBoxLayoutState& aState, nsIFrame* aBox, nscoord& aFlex)
 
     // attribute value overrides CSS
     nsIContent* content = aBox->GetContent();
-    if (content && content->IsXUL()) {
+    if (content && content->IsXULElement()) {
         nsresult error;
         nsAutoString value;
 
diff --git a/layout/xul/nsBoxFrame.cpp b/layout/xul/nsBoxFrame.cpp
index 5728c80cd5..918fbc07fd 100644
--- a/layout/xul/nsBoxFrame.cpp
+++ b/layout/xul/nsBoxFrame.cpp
@@ -1116,11 +1116,10 @@ nsBoxFrame::AttributeChanged(int32_t aNameSpaceID,
 
   // Ignore 'width', 'height', 'screenX', 'screenY' and 'sizemode' on a
   // <window>.
-  nsIAtom *tag = mContent->Tag();
-  if ((tag == nsGkAtoms::window ||
-       tag == nsGkAtoms::page ||
-       tag == nsGkAtoms::dialog ||
-       tag == nsGkAtoms::wizard) &&
+  if (mContent->IsAnyOfXULElements(nsGkAtoms::window,
+                                   nsGkAtoms::page,
+                                   nsGkAtoms::dialog,
+                                   nsGkAtoms::wizard) &&
       (nsGkAtoms::width == aAttribute ||
        nsGkAtoms::height == aAttribute ||
        nsGkAtoms::screenX == aAttribute ||
@@ -1246,7 +1245,7 @@ nsBoxFrame::AttributeChanged(int32_t aNameSpaceID,
     RegUnregAccessKey(true);
   }
   else if (aAttribute == nsGkAtoms::rows &&
-           tag == nsGkAtoms::tree) {
+           mContent->IsXULElement(nsGkAtoms::tree)) {
     // Reflow ourselves and all our children if "rows" changes, since
     // nsTreeBodyFrame's layout reads this from its parent (this frame).
     PresContext()->PresShell()->
@@ -1313,7 +1312,7 @@ nsBoxFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   mozilla::layers::FrameMetrics::ViewID scrollTargetId =
     mozilla::layers::FrameMetrics::NULL_SCROLL_ID;
 
-  if (GetContent()->IsXUL()) {
+  if (GetContent()->IsXULElement()) {
     // forcelayer is only supported on XUL elements with box layout
     if (GetContent()->HasAttr(kNameSpaceID_None, nsGkAtoms::layer)) {
       forceLayer = true;
@@ -1877,16 +1876,13 @@ nsBoxFrame::RegUnregAccessKey(bool aDoReg)
 {
   MOZ_ASSERT(mContent);
 
-  // find out what type of element this is
-  nsIAtom *atom = mContent->Tag();
-
   // only support accesskeys for the following elements
-  if (atom != nsGkAtoms::button &&
-      atom != nsGkAtoms::toolbarbutton &&
-      atom != nsGkAtoms::checkbox &&
-      atom != nsGkAtoms::textbox &&
-      atom != nsGkAtoms::tab &&
-      atom != nsGkAtoms::radio) {
+  if (!mContent->IsAnyOfXULElements(nsGkAtoms::button,
+                                    nsGkAtoms::toolbarbutton,
+                                    nsGkAtoms::checkbox,
+                                    nsGkAtoms::textbox,
+                                    nsGkAtoms::tab,
+                                    nsGkAtoms::radio)) {
     return;
   }
 
diff --git a/layout/xul/nsListBoxBodyFrame.cpp b/layout/xul/nsListBoxBodyFrame.cpp
index 32b84ee72a..e2d605c74a 100644
--- a/layout/xul/nsListBoxBodyFrame.cpp
+++ b/layout/xul/nsListBoxBodyFrame.cpp
@@ -581,7 +581,7 @@ nsListBoxBodyFrame::GetIndexOfItem(nsIDOMElement* aItem, int32_t* _retval)
     FlattenedChildIterator iter(mContent);
     for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
       // we hit a list row, count it
-      if (child->Tag() == nsGkAtoms::listitem) {
+      if (child->IsXULElement(nsGkAtoms::listitem)) {
         // is this it?
         if (child == itemContent)
           return NS_OK;
@@ -607,7 +607,7 @@ nsListBoxBodyFrame::GetItemAtIndex(int32_t aIndex, nsIDOMElement** aItem)
   FlattenedChildIterator iter(mContent);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
     // we hit a list row, check if it is the one we are looking for
-    if (child->Tag() == nsGkAtoms::listitem) {
+    if (child->IsXULElement(nsGkAtoms::listitem)) {
       // is this it?
       if (itemCount == aIndex) {
         return CallQueryInterface(child, aItem);
@@ -709,7 +709,7 @@ nsListBoxBodyFrame::ComputeIntrinsicISize(nsBoxLayoutState& aBoxLayoutState)
 
     FlattenedChildIterator iter(mContent);
     for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-      if (child->Tag() == nsGkAtoms::listitem) {
+      if (child->IsXULElement(nsGkAtoms::listitem)) {
         nsRenderingContext* rendContext = aBoxLayoutState.GetRenderingContext();
         if (rendContext) {
           nsAutoString value;
@@ -747,7 +747,7 @@ nsListBoxBodyFrame::ComputeTotalRowCount()
   mRowCount = 0;
   FlattenedChildIterator iter(mContent);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-    if (child->Tag() == nsGkAtoms::listitem) {
+    if (child->IsXULElement(nsGkAtoms::listitem)) {
       ++mRowCount;
     }
   }
@@ -1099,7 +1099,7 @@ IsListItemChild(nsListBoxBodyFrame* aParent, nsIContent* aChild,
                 nsIFrame** aChildFrame)
 {
   *aChildFrame = nullptr;
-  if (!aChild->IsXUL() || aChild->Tag() != nsGkAtoms::listitem) {
+  if (!aChild->IsXULElement(nsGkAtoms::listitem)) {
     return false;
   }
   nsIFrame* existingFrame = aChild->GetPrimaryFrame();
@@ -1463,7 +1463,7 @@ nsListBoxBodyFrame::GetListItemContentAt(int32_t aIndex, nsIContent** aContent)
   int32_t itemsFound = 0;
   FlattenedChildIterator iter(mContent);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-    if (child->Tag() == nsGkAtoms::listitem) {
+    if (child->IsXULElement(nsGkAtoms::listitem)) {
       ++itemsFound;
       if (itemsFound-1 == aIndex) {
         *aContent = child;
@@ -1482,7 +1482,7 @@ nsListBoxBodyFrame::GetListItemNextSibling(nsIContent* aListItem, nsIContent** a
   nsIContent *prevKid = nullptr;
   FlattenedChildIterator iter(mContent);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-    if (child->Tag() == nsGkAtoms::listitem) {
+    if (child->IsXULElement(nsGkAtoms::listitem)) {
       ++aSiblingIndex;
       if (prevKid == aListItem) {
         *aContent = child;
diff --git a/layout/xul/nsMenuPopupFrame.cpp b/layout/xul/nsMenuPopupFrame.cpp
index ce4cc2a4e4..90927bfab9 100644
--- a/layout/xul/nsMenuPopupFrame.cpp
+++ b/layout/xul/nsMenuPopupFrame.cpp
@@ -279,8 +279,8 @@ nsMenuPopupFrame::CreateWidgetForView(nsView* aView)
   nsTransparencyMode mode = nsLayoutUtils::GetFrameTransparency(this, this);
   nsIContent* parentContent = GetContent()->GetParent();
   nsIAtom *tag = nullptr;
-  if (parentContent)
-    tag = parentContent->Tag();
+  if (parentContent && parentContent->IsXULElement())
+    tag = parentContent->NodeInfo()->NameAtom();
   widgetData.mSupportTranslucency = mode == eTransparencyTransparent;
   widgetData.mDropShadow = !(mode == eTransparencyTransparent || tag == nsGkAtoms::menulist);
   widgetData.mPopupLevel = PopupLevel(widgetData.mNoAutoHide);
diff --git a/layout/xul/nsResizerFrame.cpp b/layout/xul/nsResizerFrame.cpp
index 2880b6a5a1..4c72e9d3bf 100644
--- a/layout/xul/nsResizerFrame.cpp
+++ b/layout/xul/nsResizerFrame.cpp
@@ -403,7 +403,7 @@ nsResizerFrame::ResizeContent(nsIContent* aContent, const Direction& aDirection,
 {
   // for XUL elements, just set the width and height attributes. For
   // other elements, set style.width and style.height
-  if (aContent->IsXUL()) {
+  if (aContent->IsXULElement()) {
     if (aOriginalSizeInfo) {
       aContent->GetAttr(kNameSpaceID_None, nsGkAtoms::width,
                         aOriginalSizeInfo->width);
diff --git a/layout/xul/nsScrollbarButtonFrame.cpp b/layout/xul/nsScrollbarButtonFrame.cpp
index 22117a8ccb..52ccaf4888 100644
--- a/layout/xul/nsScrollbarButtonFrame.cpp
+++ b/layout/xul/nsScrollbarButtonFrame.cpp
@@ -232,7 +232,7 @@ nsScrollbarButtonFrame::GetChildWithTag(nsPresContext* aPresContext,
 
     if (child) {
       // see if it is the child
-       if (child->Tag() == atom)
+       if (child->IsXULElement(atom))
        {
          result = childFrame;
 
@@ -264,7 +264,7 @@ nsScrollbarButtonFrame::GetParentWithTag(nsIAtom* toFind, nsIFrame* start,
         // get the content node
         nsIContent* child = start->GetContent();
 
-        if (child && child->Tag() == toFind) {
+        if (child && child->IsXULElement(toFind)) {
           result = start;
           return NS_OK;
         }
diff --git a/layout/xul/nsXULPopupManager.cpp b/layout/xul/nsXULPopupManager.cpp
index 9a83789786..f9ca34e225 100644
--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -1569,7 +1569,7 @@ nsXULPopupManager::GetLastTriggerNode(nsIDocument* aDocument, bool aIsTooltip)
   // fired. In this case, just use the cached node, as the popup is not yet in
   // the list of open popups.
   if (mOpeningPopup && mOpeningPopup->GetCurrentDoc() == aDocument &&
-      aIsTooltip == (mOpeningPopup->Tag() == nsGkAtoms::tooltip)) {
+      aIsTooltip == mOpeningPopup->IsXULElement(nsGkAtoms::tooltip)) {
     node = do_QueryInterface(nsMenuPopupFrame::GetTriggerContent(GetPopupFrameForContent(mOpeningPopup, false)));
   }
   else {
@@ -1833,13 +1833,13 @@ nsXULPopupManager::UpdateMenuItems(nsIContent* aPopup)
   for (nsCOMPtr<nsIContent> grandChild = aPopup->GetFirstChild();
        grandChild;
        grandChild = grandChild->GetNextSibling()) {
-    if (grandChild->IsXUL(nsGkAtoms::menugroup)) {
+    if (grandChild->IsXULElement(nsGkAtoms::menugroup)) {
       if (grandChild->GetChildCount() == 0) {
         continue;
       }
       grandChild = grandChild->GetFirstChild();
     }
-    if (grandChild->IsXUL(nsGkAtoms::menuitem)) {
+    if (grandChild->IsXULElement(nsGkAtoms::menuitem)) {
       // See if we have a command attribute.
       nsAutoString command;
       grandChild->GetAttr(kNameSpaceID_None, nsGkAtoms::command, command);
@@ -1873,7 +1873,7 @@ nsXULPopupManager::UpdateMenuItems(nsIContent* aPopup)
       }
     }
     if (!grandChild->GetNextSibling() &&
-        grandChild->GetParent()->IsXUL(nsGkAtoms::menugroup)) {
+        grandChild->GetParent()->IsXULElement(nsGkAtoms::menugroup)) {
       grandChild = grandChild->GetParent();
     }
   }
@@ -2241,7 +2241,7 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
   if (aStart) {
     if (aStart->GetNextSibling())
       currFrame = aStart->GetNextSibling();
-    else if (aStart->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+    else if (aStart->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = aStart->GetParent()->GetNextSibling();
   }
   else
@@ -2253,11 +2253,11 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
     if (IsValidMenuItem(presContext, currFrameContent, aIsPopup)) {
       return do_QueryFrame(currFrame);
     }
-    if (currFrameContent->IsXUL(nsGkAtoms::menugroup) &&
+    if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0)
       currFrame = currFrame->GetFirstPrincipalChild();
     else if (!currFrame->GetNextSibling() &&
-             currFrame->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+             currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetNextSibling();
     else
       currFrame = currFrame->GetNextSibling();
@@ -2272,11 +2272,11 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
     if (IsValidMenuItem(presContext, currFrameContent, aIsPopup)) {
       return do_QueryFrame(currFrame);
     }
-    if (currFrameContent->IsXUL(nsGkAtoms::menugroup) &&
+    if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0)
       currFrame = currFrame->GetFirstPrincipalChild();
     else if (!currFrame->GetNextSibling() &&
-             currFrame->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+             currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetNextSibling();
     else
       currFrame = currFrame->GetNextSibling();
@@ -2304,7 +2304,7 @@ nsXULPopupManager::GetPreviousMenuItem(nsContainerFrame* aParent,
   if (aStart) {
     if (aStart->GetPrevSibling())
       currFrame = aStart->GetPrevSibling();
-    else if (aStart->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+    else if (aStart->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = aStart->GetParent()->GetPrevSibling();
   }
   else
@@ -2316,13 +2316,13 @@ nsXULPopupManager::GetPreviousMenuItem(nsContainerFrame* aParent,
     if (IsValidMenuItem(presContext, currFrameContent, aIsPopup)) {
       return do_QueryFrame(currFrame);
     }
-    if (currFrameContent->IsXUL(nsGkAtoms::menugroup) &&
+    if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0) {
       const nsFrameList& menugroupFrames(currFrame->PrincipalChildList());
       currFrame = menugroupFrames.LastChild();
     }
     else if (!currFrame->GetPrevSibling() &&
-             currFrame->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+             currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetPrevSibling();
     else
       currFrame = currFrame->GetPrevSibling();
@@ -2337,13 +2337,13 @@ nsXULPopupManager::GetPreviousMenuItem(nsContainerFrame* aParent,
     if (IsValidMenuItem(presContext, currFrameContent, aIsPopup)) {
       return do_QueryFrame(currFrame);
     }
-    if (currFrameContent->IsXUL(nsGkAtoms::menugroup) &&
+    if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0) {
       const nsFrameList& menugroupFrames(currFrame->PrincipalChildList());
       currFrame = menugroupFrames.LastChild();
     }
     else if (!currFrame->GetPrevSibling() &&
-             currFrame->GetParent()->GetContent()->IsXUL(nsGkAtoms::menugroup))
+             currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetPrevSibling();
     else
       currFrame = currFrame->GetPrevSibling();
@@ -2358,13 +2358,12 @@ nsXULPopupManager::IsValidMenuItem(nsPresContext* aPresContext,
                                    nsIContent* aContent,
                                    bool aOnPopup)
 {
-  int32_t ns = aContent->GetNameSpaceID();
-  nsIAtom *tag = aContent->Tag();
-  if (ns == kNameSpaceID_XUL) {
-    if (tag != nsGkAtoms::menu && tag != nsGkAtoms::menuitem)
+  if (aContent->IsXULElement()) {
+    if (!aContent->IsAnyOfXULElements(nsGkAtoms::menu, nsGkAtoms::menuitem)) {
       return false;
+    }
   }
-  else if (ns != kNameSpaceID_XHTML || !aOnPopup || tag != nsGkAtoms::option) {
+  else if (!aOnPopup || !aContent->IsHTMLElement(nsGkAtoms::option)) {
     return false;
   }
 
diff --git a/layout/xul/nsXULTooltipListener.cpp b/layout/xul/nsXULTooltipListener.cpp
index 4624df7164..0f25c5845c 100644
--- a/layout/xul/nsXULTooltipListener.cpp
+++ b/layout/xul/nsXULTooltipListener.cpp
@@ -158,7 +158,7 @@ nsXULTooltipListener::MouseMove(nsIDOMEvent* aEvent)
     aEvent->InternalDOMEvent()->GetCurrentTarget());
   mSourceNode = do_GetWeakReference(sourceContent);
 #ifdef MOZ_XUL
-  mIsSourceTree = sourceContent->Tag() == nsGkAtoms::treechildren;
+  mIsSourceTree = sourceContent->IsXULElement(nsGkAtoms::treechildren);
   if (mIsSourceTree)
     CheckTreeBodyMove(mouseEvent);
 #endif
@@ -182,11 +182,9 @@ nsXULTooltipListener::MouseMove(nsIDOMEvent* aEvent)
                                     nsGkAtoms::_true, eCaseMatters)) {
       nsCOMPtr<nsIContent> targetContent = do_QueryInterface(eventTarget);
       while (targetContent && targetContent != sourceContent) {
-        nsIAtom* tag = targetContent->Tag();
-        if (targetContent->GetNameSpaceID() == kNameSpaceID_XUL &&
-            (tag == nsGkAtoms::menupopup ||
-             tag == nsGkAtoms::panel ||
-             tag == nsGkAtoms::tooltip)) {
+        if (targetContent->IsAnyOfXULElements(nsGkAtoms::menupopup,
+                                              nsGkAtoms::panel,
+                                              nsGkAtoms::tooltip)) {
           mSourceNode = nullptr;
           return;
         }
@@ -539,7 +537,7 @@ GetImmediateChild(nsIContent* aContent, nsIAtom *aTag, nsIContent** aResult)
   for (uint32_t i = 0; i < childCount; i++) {
     nsIContent *child = aContent->GetChildAt(i);
 
-    if (child->Tag() == aTag) {
+    if (child->IsXULElement(aTag)) {
       *aResult = child;
       NS_ADDREF(*aResult);
       return;
diff --git a/layout/xul/tree/nsTreeBodyFrame.cpp b/layout/xul/tree/nsTreeBodyFrame.cpp
index fad899fea5..e3a5122838 100644
--- a/layout/xul/tree/nsTreeBodyFrame.cpp
+++ b/layout/xul/tree/nsTreeBodyFrame.cpp
@@ -194,8 +194,7 @@ nsTreeBodyFrame::GetMinSize(nsBoxLayoutState& aBoxLayoutState)
   if (MOZ_UNLIKELY(!baseElement)) {
     desiredRows = 0;
   }
-  else if (baseElement->Tag() == nsGkAtoms::select &&
-           baseElement->IsHTML()) {
+  else if (baseElement->IsHTMLElement(nsGkAtoms::select)) {
     min.width = CalcMaxRowWidth();
     nsAutoString size;
     baseElement->GetAttr(kNameSpaceID_None, nsGkAtoms::size, size);
@@ -1786,8 +1785,7 @@ nsTreeBodyFrame::MarkDirtyIfSelect()
 {
   nsIContent* baseElement = GetBaseElement();
 
-  if (baseElement && baseElement->Tag() == nsGkAtoms::select &&
-      baseElement->IsHTML()) {
+  if (baseElement && baseElement->IsHTMLElement(nsGkAtoms::select)) {
     // If we are an intrinsically sized select widget, we may need to
     // resize, if the widest item was removed or a new item was added.
     // XXX optimize this more
@@ -4319,7 +4317,7 @@ nsTreeBodyFrame::GetBaseElement()
 
       if (ni->Equals(nsGkAtoms::tree, kNameSpaceID_XUL) ||
           (ni->Equals(nsGkAtoms::select) &&
-           content->IsHTML()))
+           content->IsHTMLElement()))
         return content;
     }
 
diff --git a/layout/xul/tree/nsTreeContentView.cpp b/layout/xul/tree/nsTreeContentView.cpp
index 09c92b1424..c1e93625f2 100644
--- a/layout/xul/tree/nsTreeContentView.cpp
+++ b/layout/xul/tree/nsTreeContentView.cpp
@@ -449,8 +449,7 @@ nsTreeContentView::GetCellText(int32_t aRow, nsITreeColumn* aCol, nsAString& _re
       && !_retval.IsEmpty())
     return NS_OK;
 
-  nsIAtom *rowTag = row->mContent->Tag();
-  if (rowTag == nsGkAtoms::treeitem && row->mContent->IsXUL()) {
+  if (row->mContent->IsXULElement(nsGkAtoms::treeitem)) {
     nsIContent* realRow =
       nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
     if (realRow) {
@@ -723,7 +722,6 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
 
   // Make sure this notification concerns us.
   // First check the tag to see if it's one that we care about.
-  nsIAtom* tag = aElement->Tag();
 
   if (mBoxObject && (aElement == mRoot || aElement == mBody)) {
     mBoxObject->ClearStyleAndImageCaches();
@@ -732,15 +730,15 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
 
   // We don't consider non-XUL nodes.
   nsIContent* parent = nullptr;
-  if (!aElement->IsXUL() ||
-      ((parent = aElement->GetParent()) && !parent->IsXUL())) {
+  if (!aElement->IsXULElement() ||
+      ((parent = aElement->GetParent()) && !parent->IsXULElement())) {
     return;
   }
-  if (tag != nsGkAtoms::treecol &&
-      tag != nsGkAtoms::treeitem &&
-      tag != nsGkAtoms::treeseparator &&
-      tag != nsGkAtoms::treerow &&
-      tag != nsGkAtoms::treecell) {
+  if (!aElement->IsAnyOfXULElements(nsGkAtoms::treecol,
+                                    nsGkAtoms::treeitem,
+                                    nsGkAtoms::treeseparator,
+                                    nsGkAtoms::treerow,
+                                    nsGkAtoms::treecell)) {
     return;
   }
 
@@ -750,14 +748,14 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
   for (nsIContent* element = aElement; element != mBody; element = element->GetParent()) {
     if (!element)
       return; // this is not for us
-    nsIAtom *parentTag = element->Tag();
-    if (element->IsXUL() && parentTag == nsGkAtoms::tree)
+    if (element->IsXULElement(nsGkAtoms::tree))
       return; // this is not for us
   }
 
   // Handle changes of the hidden attribute.
   if (aAttribute == nsGkAtoms::hidden &&
-     (tag == nsGkAtoms::treeitem || tag == nsGkAtoms::treeseparator)) {
+      aElement->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                   nsGkAtoms::treeseparator)) {
     bool hidden = aElement->AttrValueIs(kNameSpaceID_None,
                                           nsGkAtoms::hidden,
                                           nsGkAtoms::_true, eCaseMatters);
@@ -780,7 +778,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
     return;
   }
 
-  if (tag == nsGkAtoms::treecol) {
+  if (aElement->IsXULElement(nsGkAtoms::treecol)) {
     if (aAttribute == nsGkAtoms::properties) {
       if (mBoxObject) {
         nsCOMPtr<nsITreeColumns> cols;
@@ -794,7 +792,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
       }
     }
   }
-  else if (tag == nsGkAtoms::treeitem) {
+  else if (aElement->IsXULElement(nsGkAtoms::treeitem)) {
     int32_t index = FindContent(aElement);
     if (index >= 0) {
       Row* row = mRows[index];
@@ -826,7 +824,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
       }
     }
   }
-  else if (tag == nsGkAtoms::treeseparator) {
+  else if (aElement->IsXULElement(nsGkAtoms::treeseparator)) {
     int32_t index = FindContent(aElement);
     if (index >= 0) {
       if (aAttribute == nsGkAtoms::properties && mBoxObject) {
@@ -834,7 +832,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
       }
     }
   }
-  else if (tag == nsGkAtoms::treerow) {
+  else if (aElement->IsXULElement(nsGkAtoms::treerow)) {
     if (aAttribute == nsGkAtoms::properties) {
       nsCOMPtr<nsIContent> parent = aElement->GetParent();
       if (parent) {
@@ -845,7 +843,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
       }
     }
   }
-  else if (tag == nsGkAtoms::treecell) {
+  else if (aElement->IsXULElement(nsGkAtoms::treecell)) {
     if (aAttribute == nsGkAtoms::ref ||
         aAttribute == nsGkAtoms::properties ||
         aAttribute == nsGkAtoms::mode ||
@@ -855,7 +853,7 @@ nsTreeContentView::AttributeChanged(nsIDocument*  aDocument,
       nsIContent* parent = aElement->GetParent();
       if (parent) {
         nsCOMPtr<nsIContent> grandParent = parent->GetParent();
-        if (grandParent && grandParent->IsXUL()) {
+        if (grandParent && grandParent->IsXULElement()) {
           int32_t index = FindContent(grandParent);
           if (index >= 0 && mBoxObject) {
             // XXX Should we make an effort to invalidate only cell ?
@@ -889,16 +887,16 @@ nsTreeContentView::ContentInserted(nsIDocument *aDocument,
 
   // Make sure this notification concerns us.
   // First check the tag to see if it's one that we care about.
-  nsIAtom *childTag = aChild->Tag();
 
   // Don't allow non-XUL nodes.
-  if (!aChild->IsXUL() || !aContainer->IsXUL())
+  if (!aChild->IsXULElement() || !aContainer->IsXULElement())
     return;
-  if (childTag != nsGkAtoms::treeitem &&
-      childTag != nsGkAtoms::treeseparator &&
-      childTag != nsGkAtoms::treechildren &&
-      childTag != nsGkAtoms::treerow &&
-      childTag != nsGkAtoms::treecell) {
+
+  if (!aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                  nsGkAtoms::treeseparator,
+                                  nsGkAtoms::treechildren,
+                                  nsGkAtoms::treerow,
+                                  nsGkAtoms::treecell)) {
     return;
   }
 
@@ -908,15 +906,14 @@ nsTreeContentView::ContentInserted(nsIDocument *aDocument,
   for (nsIContent* element = aContainer; element != mBody; element = element->GetParent()) {
     if (!element)
       return; // this is not for us
-    nsIAtom *parentTag = element->Tag();
-    if (element->IsXUL() && parentTag == nsGkAtoms::tree)
+    if (element->IsXULElement(nsGkAtoms::tree))
       return; // this is not for us
   }
 
   // Lots of codepaths under here that do all sorts of stuff, so be safe.
   nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
 
-  if (childTag == nsGkAtoms::treechildren) {
+  if (aChild->IsXULElement(nsGkAtoms::treechildren)) {
     int32_t index = FindContent(aContainer);
     if (index >= 0) {
       Row* row = mRows[index];
@@ -930,16 +927,16 @@ nsTreeContentView::ContentInserted(nsIDocument *aDocument,
       }
     }
   }
-  else if (childTag == nsGkAtoms::treeitem ||
-           childTag == nsGkAtoms::treeseparator) {
+  else if (aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                      nsGkAtoms::treeseparator)) {
     InsertRowFor(aContainer, aChild);
   }
-  else if (childTag == nsGkAtoms::treerow) {
+  else if (aChild->IsXULElement(nsGkAtoms::treerow)) {
     int32_t index = FindContent(aContainer);
     if (index >= 0 && mBoxObject)
       mBoxObject->InvalidateRow(index);
   }
-  else if (childTag == nsGkAtoms::treecell) {
+  else if (aChild->IsXULElement(nsGkAtoms::treecell)) {
     nsCOMPtr<nsIContent> parent = aContainer->GetParent();
     if (parent) {
       int32_t index = FindContent(parent);
@@ -960,16 +957,16 @@ nsTreeContentView::ContentRemoved(nsIDocument *aDocument,
 
   // Make sure this notification concerns us.
   // First check the tag to see if it's one that we care about.
-  nsIAtom *tag = aChild->Tag();
 
   // We don't consider non-XUL nodes.
-  if (!aChild->IsXUL() || !aContainer->IsXUL())
+  if (!aChild->IsXULElement() || !aContainer->IsXULElement())
     return;
-  if (tag != nsGkAtoms::treeitem &&
-      tag != nsGkAtoms::treeseparator &&
-      tag != nsGkAtoms::treechildren &&
-      tag != nsGkAtoms::treerow &&
-      tag != nsGkAtoms::treecell) {
+
+  if (!aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                  nsGkAtoms::treeseparator,
+                                  nsGkAtoms::treechildren,
+                                  nsGkAtoms::treerow,
+                                  nsGkAtoms::treecell)) {
     return;
   }
 
@@ -979,15 +976,14 @@ nsTreeContentView::ContentRemoved(nsIDocument *aDocument,
   for (nsIContent* element = aContainer; element != mBody; element = element->GetParent()) {
     if (!element)
       return; // this is not for us
-    nsIAtom *parentTag = element->Tag();
-    if (element->IsXUL() && parentTag == nsGkAtoms::tree)
+    if (element->IsXULElement(nsGkAtoms::tree))
       return; // this is not for us
   }
 
   // Lots of codepaths under here that do all sorts of stuff, so be safe.
   nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
 
-  if (tag == nsGkAtoms::treechildren) {
+  if (aChild->IsXULElement(nsGkAtoms::treechildren)) {
     int32_t index = FindContent(aContainer);
     if (index >= 0) {
       Row* row = mRows[index];
@@ -1000,9 +996,8 @@ nsTreeContentView::ContentRemoved(nsIDocument *aDocument,
       }
     }
   }
-  else if (tag == nsGkAtoms::treeitem ||
-           tag == nsGkAtoms::treeseparator
-          ) {
+  else if (aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                      nsGkAtoms::treeseparator)) {
     int32_t index = FindContent(aChild);
     if (index >= 0) {
       int32_t count = RemoveRow(index);
@@ -1010,12 +1005,12 @@ nsTreeContentView::ContentRemoved(nsIDocument *aDocument,
         mBoxObject->RowCountChanged(index, -count);
     }
   }
-  else if (tag == nsGkAtoms::treerow) {
+  else if (aChild->IsXULElement(nsGkAtoms::treerow)) {
     int32_t index = FindContent(aContainer);
     if (index >= 0 && mBoxObject)
       mBoxObject->InvalidateRow(index);
   }
-  else if (tag == nsGkAtoms::treecell) {
+  else if (aChild->IsXULElement(nsGkAtoms::treecell)) {
     nsCOMPtr<nsIContent> parent = aContainer->GetParent();
     if (parent) {
       int32_t index = FindContent(parent);
@@ -1040,20 +1035,19 @@ nsTreeContentView::Serialize(nsIContent* aContent, int32_t aParentIndex,
                              int32_t* aIndex, nsTArray<nsAutoPtr<Row> >& aRows)
 {
   // Don't allow non-XUL nodes.
-  if (!aContent->IsXUL())
+  if (!aContent->IsXULElement())
     return;
 
   dom::FlattenedChildIterator iter(aContent);
   for (nsIContent* content = iter.GetNextChild(); content; content = iter.GetNextChild()) {
-    nsIAtom *tag = content->Tag();
     int32_t count = aRows.Length();
 
-    if (content->IsXUL()) {
-      if (tag == nsGkAtoms::treeitem)
-        SerializeItem(content, aParentIndex, aIndex, aRows);
-      else if (tag == nsGkAtoms::treeseparator)
-        SerializeSeparator(content, aParentIndex, aIndex, aRows);
+    if (content->IsXULElement(nsGkAtoms::treeitem)) {
+      SerializeItem(content, aParentIndex, aIndex, aRows);
+    } else if (content->IsXULElement(nsGkAtoms::treeseparator)) {
+      SerializeSeparator(content, aParentIndex, aIndex, aRows);
     }
+
     *aIndex += aRows.Length() - count;
   }
 }
@@ -1077,7 +1071,7 @@ nsTreeContentView::SerializeItem(nsIContent* aContent, int32_t aParentIndex,
       row->SetOpen(true);
       nsIContent* child =
         nsTreeUtils::GetImmediateChild(aContent, nsGkAtoms::treechildren);
-      if (child && child->IsXUL()) {
+      if (child && child->IsXULElement()) {
         // Now, recursively serialize our child.
         int32_t count = aRows.Length();
         int32_t index = 0;
@@ -1113,7 +1107,7 @@ nsTreeContentView::GetIndexInSubtree(nsIContent* aContainer,
 {
   uint32_t childCount = aContainer->GetChildCount();
   
-  if (!aContainer->IsXUL())
+  if (!aContainer->IsXULElement())
     return;
 
   for (uint32_t i = 0; i < childCount; i++) {
@@ -1122,29 +1116,25 @@ nsTreeContentView::GetIndexInSubtree(nsIContent* aContainer,
     if (content == aContent)
       break;
 
-    nsIAtom *tag = content->Tag();
-
-    if (content->IsXUL()) {
-      if (tag == nsGkAtoms::treeitem) {
-        if (! content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
-                                   nsGkAtoms::_true, eCaseMatters)) {
-          (*aIndex)++;
-          if (content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
-                                   nsGkAtoms::_true, eCaseMatters) &&
-              content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
-                                   nsGkAtoms::_true, eCaseMatters)) {
-            nsIContent* child =
-              nsTreeUtils::GetImmediateChild(content, nsGkAtoms::treechildren);
-            if (child && child->IsXUL())
-              GetIndexInSubtree(child, aContent, aIndex);
-          }
+    if (content->IsXULElement(nsGkAtoms::treeitem)) {
+      if (! content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                                 nsGkAtoms::_true, eCaseMatters)) {
+        (*aIndex)++;
+        if (content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
+                                 nsGkAtoms::_true, eCaseMatters) &&
+            content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
+                                 nsGkAtoms::_true, eCaseMatters)) {
+          nsIContent* child =
+            nsTreeUtils::GetImmediateChild(content, nsGkAtoms::treechildren);
+          if (child && child->IsXULElement())
+            GetIndexInSubtree(child, aContent, aIndex);
         }
       }
-      else if (tag == nsGkAtoms::treeseparator) {
-        if (! content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
-                                   nsGkAtoms::_true, eCaseMatters))
-          (*aIndex)++;
-      }
+    }
+    else if (content->IsXULElement(nsGkAtoms::treeseparator)) {
+      if (! content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                                 nsGkAtoms::_true, eCaseMatters))
+        (*aIndex)++;
     }
   }
 }
@@ -1156,7 +1146,7 @@ nsTreeContentView::EnsureSubtree(int32_t aIndex)
 
   nsIContent* child;
   child = nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treechildren);
-  if (!child || !child->IsXUL()) {
+  if (!child || !child->IsXULElement()) {
     return 0;
   }
 
@@ -1204,9 +1194,8 @@ nsTreeContentView::InsertRowFor(nsIContent* aParent, nsIContent* aChild)
   bool insertRow = false;
 
   nsCOMPtr<nsIContent> grandParent = aParent->GetParent();
-  nsIAtom* grandParentTag = grandParent->Tag();
 
-  if (grandParent->IsXUL() && grandParentTag == nsGkAtoms::tree) {
+  if (grandParent->IsXULElement(nsGkAtoms::tree)) {
     // Allow insertion to the outermost container.
     insertRow = true;
   }
@@ -1237,12 +1226,10 @@ int32_t
 nsTreeContentView::InsertRow(int32_t aParentIndex, int32_t aIndex, nsIContent* aContent)
 {
   nsAutoTArray<nsAutoPtr<Row>, 8> rows;
-  nsIAtom *tag = aContent->Tag();
-  if (aContent->IsXUL()) {
-    if (tag == nsGkAtoms::treeitem)
-      SerializeItem(aContent, aParentIndex, &aIndex, rows);
-    else if (tag == nsGkAtoms::treeseparator)
-      SerializeSeparator(aContent, aParentIndex, &aIndex, rows);
+  if (aContent->IsXULElement(nsGkAtoms::treeitem)) {
+    SerializeItem(aContent, aParentIndex, &aIndex, rows);
+  } else if (aContent->IsXULElement(nsGkAtoms::treeseparator)) {
+    SerializeSeparator(aContent, aParentIndex, &aIndex, rows);
   }
 
   // We can't use InsertElementsAt since the destination can't steal
@@ -1365,7 +1352,7 @@ nsTreeContentView::GetCell(nsIContent* aContainer, nsITreeColumn* aCol)
   int32_t j = 0;
   dom::FlattenedChildIterator iter(aContainer);
   for (nsIContent* cell = iter.GetNextChild(); cell; cell = iter.GetNextChild()) {
-    if (cell->Tag() == nsGkAtoms::treecell) {
+    if (cell->IsXULElement(nsGkAtoms::treecell)) {
       if (colAtom && cell->AttrValueIs(kNameSpaceID_None, nsGkAtoms::ref,
                                        colAtom, eCaseMatters)) {
         result = cell;
diff --git a/layout/xul/tree/nsTreeUtils.cpp b/layout/xul/tree/nsTreeUtils.cpp
index b65ebcf226..e5d0dee315 100644
--- a/layout/xul/tree/nsTreeUtils.cpp
+++ b/layout/xul/tree/nsTreeUtils.cpp
@@ -56,7 +56,7 @@ nsTreeUtils::GetImmediateChild(nsIContent* aContainer, nsIAtom* aTag)
 {
   dom::FlattenedChildIterator iter(aContainer);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-    if (child->Tag() == aTag) {
+    if (child->IsXULElement(aTag)) {
       return child;
     }
   }
@@ -69,7 +69,7 @@ nsTreeUtils::GetDescendantChild(nsIContent* aContainer, nsIAtom* aTag)
 {
   dom::FlattenedChildIterator iter(aContainer);
   for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
-    if (child->Tag() == aTag) {
+    if (child->IsXULElement(aTag)) {
       return child;
     }
 
diff --git a/parser/html/nsHtml5DocumentBuilder.cpp b/parser/html/nsHtml5DocumentBuilder.cpp
index 08e4d8a1b6..60d5ba729f 100644
--- a/parser/html/nsHtml5DocumentBuilder.cpp
+++ b/parser/html/nsHtml5DocumentBuilder.cpp
@@ -81,7 +81,7 @@ nsHtml5DocumentBuilder::UpdateStyleSheet(nsIContent* aElement)
     mScriptLoader->AddExecuteBlocker();
   }
 
-  if (aElement->IsHTML(nsGkAtoms::link)) {
+  if (aElement->IsHTMLElement(nsGkAtoms::link)) {
     // look for <link rel="next" href="url">
     nsAutoString relVal;
     aElement->GetAttr(kNameSpaceID_None, nsGkAtoms::rel, relVal);
diff --git a/parser/html/nsHtml5TreeBuilderCppSupplement.h b/parser/html/nsHtml5TreeBuilderCppSupplement.h
index 9e497c1f19..faecc546a6 100644
--- a/parser/html/nsHtml5TreeBuilderCppSupplement.h
+++ b/parser/html/nsHtml5TreeBuilderCppSupplement.h
@@ -1146,7 +1146,7 @@ nsHtml5TreeBuilder::getFormPointerForContext(nsIContentHandle* aContext)
   // form pointer. This traversal is why aContext must not be an emtpy handle.
   nsIContent* nearestForm = nullptr;
   while (currentAncestor) {
-    if (currentAncestor->IsHTML(nsGkAtoms::form)) {
+    if (currentAncestor->IsHTMLElement(nsGkAtoms::form)) {
       nearestForm = currentAncestor;
       break;
     }
diff --git a/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp b/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
index 8f583dc596..03abd19adc 100644
--- a/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
+++ b/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
@@ -884,7 +884,7 @@ nsTypeAheadFind::RangeStartsInsideLink(nsIDOMRange *aRange,
     // Keep testing while startContent is equal to something,
     // eventually we'll run out of ancestors
 
-    if (startContent->IsHTML()) {
+    if (startContent->IsHTMLElement()) {
       nsCOMPtr<mozilla::dom::Link> link(do_QueryInterface(startContent));
       if (link) {
         // Check to see if inside HTML link
diff --git a/widget/cocoa/nsMenuBarX.mm b/widget/cocoa/nsMenuBarX.mm
index c9654db48e..1c036abd04 100644
--- a/widget/cocoa/nsMenuBarX.mm
+++ b/widget/cocoa/nsMenuBarX.mm
@@ -132,8 +132,7 @@ void nsMenuBarX::ConstructNativeMenus()
   for (uint32_t i = 0; i < count; i++) { 
     nsIContent *menuContent = mContent->GetChildAt(i);
     if (menuContent &&
-        menuContent->Tag() == nsGkAtoms::menu &&
-        menuContent->IsXUL()) {
+        menuContent->IsXULElement(nsGkAtoms::menu)) {
       nsMenuX* newMenu = new nsMenuX();
       if (newMenu) {
         nsresult rv = newMenu->Create(this, this, menuContent);
diff --git a/widget/cocoa/nsMenuX.mm b/widget/cocoa/nsMenuX.mm
index dcb7bf6704..566dfcde77 100644
--- a/widget/cocoa/nsMenuX.mm
+++ b/widget/cocoa/nsMenuX.mm
@@ -432,11 +432,12 @@ void nsMenuX::MenuConstruct()
     nsIContent *child = menuPopup->GetChildAt(i);
     if (child) {
       // depending on the type, create a menu item, separator, or submenu
-      nsIAtom *tag = child->Tag();
-      if (tag == nsGkAtoms::menuitem || tag == nsGkAtoms::menuseparator)
+      if (child->IsAnyOfXULElements(nsGkAtoms::menuitem,
+                                    nsGkAtoms::menuseparator)) {
         LoadMenuItem(child);
-      else if (tag == nsGkAtoms::menu)
+      } else if (child->IsXULElement(nsGkAtoms::menu)) {
         LoadSubMenu(child);
+      }
     }
   } // for each menu item
 
@@ -501,7 +502,7 @@ void nsMenuX::LoadMenuItem(nsIContent* inMenuItemContent)
   // printf("menuitem %s \n", NS_LossyConvertUTF16toASCII(menuitemName).get());
 
   EMenuItemType itemType = eRegularMenuItemType;
-  if (inMenuItemContent->Tag() == nsGkAtoms::menuseparator) {
+  if (inMenuItemContent->IsXULElement(nsGkAtoms::menuseparator)) {
     itemType = eSeparatorMenuItemType;
   }
   else {
diff --git a/widget/cocoa/nsNativeThemeCocoa.mm b/widget/cocoa/nsNativeThemeCocoa.mm
index ff1d2883cb..bf2f2677e5 100644
--- a/widget/cocoa/nsNativeThemeCocoa.mm
+++ b/widget/cocoa/nsNativeThemeCocoa.mm
@@ -280,9 +280,9 @@ static BOOL IsToolbarStyleContainer(nsIFrame* aFrame)
   if (!content)
     return NO;
 
-  if (content->Tag() == nsGkAtoms::toolbar ||
-      content->Tag() == nsGkAtoms::toolbox ||
-      content->Tag() == nsGkAtoms::statusbar)
+  if (content->IsAnyOfXULElements(nsGkAtoms::toolbar,
+                                  nsGkAtoms::toolbox,
+                                  nsGkAtoms::statusbar))
     return YES;
 
   switch (aFrame->StyleDisplay()->mAppearance) {
@@ -1741,7 +1741,7 @@ nsNativeThemeCocoa::DrawMeter(CGContextRef cgContext, const HIRect& inBoxRect,
   // When using -moz-meterbar on an non meter element, we will not be able to
   // get all the needed information so we just draw an empty meter.
   nsIContent* content = aFrame->GetContent();
-  if (!(content && content->IsHTML(nsGkAtoms::meter))) {
+  if (!(content && content->IsHTMLElement(nsGkAtoms::meter))) {
     DrawCellWithSnapping(mMeterBarCell, cgContext, inBoxRect,
                          meterSetting, VerticalAlignFactor(aFrame),
                          mCellDrawView, IsFrameRTL(aFrame));
@@ -2514,7 +2514,7 @@ nsNativeThemeCocoa::DrawWidgetBackground(nsRenderingContext* aContext,
 
     case NS_THEME_SPINNER: {
       nsIContent* content = aFrame->GetContent();
-      if (content->IsHTML()) {
+      if (content->IsHTMLElement()) {
         // In HTML the theming for the spin buttons is drawn individually into
         // their own backgrounds instead of being drawn into the background of
         // their spinner parent as it is for XUL.
@@ -2634,7 +2634,7 @@ nsNativeThemeCocoa::DrawWidgetBackground(nsRenderingContext* aContext,
       // though, check the focused attribute of xul textboxes in this case.
       // On Mac, focus rings are always shown for textboxes, so we do not need
       // to check the window's focus ring state here
-      if (aFrame->GetContent()->IsXUL() && IsFocused(aFrame)) {
+      if (aFrame->GetContent()->IsXULElement() && IsFocused(aFrame)) {
         eventState |= NS_EVENT_STATE_FOCUS;
       }
 
@@ -3206,7 +3206,7 @@ nsNativeThemeCocoa::GetMinimumWidgetSize(nsPresContext* aPresContext,
     case NS_THEME_SPINNER_DOWN_BUTTON:
     {
       SInt32 buttonHeight = 0, buttonWidth = 0;
-      if (aFrame->GetContent()->IsXUL()) {
+      if (aFrame->GetContent()->IsXULElement()) {
         ::GetThemeMetric(kThemeMetricLittleArrowsWidth, &buttonWidth);
         ::GetThemeMetric(kThemeMetricLittleArrowsHeight, &buttonHeight);
       } else {
diff --git a/widget/cocoa/nsStandaloneNativeMenu.mm b/widget/cocoa/nsStandaloneNativeMenu.mm
index 490c9eec72..98a5fd8f6f 100644
--- a/widget/cocoa/nsStandaloneNativeMenu.mm
+++ b/widget/cocoa/nsStandaloneNativeMenu.mm
@@ -37,9 +37,7 @@ nsStandaloneNativeMenu::Init(nsIDOMElement * aDOMElement)
   nsCOMPtr<nsIContent> content = do_QueryInterface(aDOMElement, &rv);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  nsIAtom * tag = content->Tag();
-  if (!content->IsXUL() ||
-      (tag != nsGkAtoms::menu && tag != nsGkAtoms::menupopup))
+  if (!content->IsAnyOfXULElements(nsGkAtoms::menu, nsGkAtoms::menupopup))
     return NS_ERROR_FAILURE;
 
   rv = nsMenuGroupOwnerX::Create(content);
diff --git a/widget/nsNativeTheme.cpp b/widget/nsNativeTheme.cpp
index 090c6286b9..e691e28a37 100644
--- a/widget/nsNativeTheme.cpp
+++ b/widget/nsNativeTheme.cpp
@@ -61,7 +61,7 @@ nsNativeTheme::GetContentState(nsIFrame* aFrame, uint8_t aWidgetType)
   bool isXULCheckboxRadio = 
     (aWidgetType == NS_THEME_CHECKBOX ||
      aWidgetType == NS_THEME_RADIO) &&
-    aFrame->GetContent()->IsXUL();
+    aFrame->GetContent()->IsXULElement();
   if (isXULCheckboxRadio)
     aFrame = aFrame->GetParent();
 
@@ -80,7 +80,7 @@ nsNativeTheme::GetContentState(nsIFrame* aFrame, uint8_t aWidgetType)
     // <input type=number> needs special handling since its nested native
     // anonymous <input type=text> takes focus for it.
     if (aWidgetType == NS_THEME_NUMBER_INPUT &&
-        frameContent->IsHTML(nsGkAtoms::input)) {
+        frameContent->IsHTMLElement(nsGkAtoms::input)) {
       nsNumberControlFrame *numberControlFrame = do_QueryFrame(aFrame);
       if (numberControlFrame && numberControlFrame->IsFocused()) {
         flags |= NS_EVENT_STATE_FOCUS;
@@ -140,7 +140,7 @@ nsNativeTheme::CheckBooleanAttr(nsIFrame* aFrame, nsIAtom* aAtom)
   if (!content)
     return false;
 
-  if (content->IsHTML())
+  if (content->IsHTMLElement())
     return content->HasAttr(kNameSpaceID_None, aAtom);
 
   // For XML/XUL elements, an attribute must be equal to the literal
@@ -173,7 +173,7 @@ nsNativeTheme::GetProgressValue(nsIFrame* aFrame)
 {
   // When we are using the HTML progress element,
   // we can get the value from the IDL property.
-  if (aFrame && aFrame->GetContent()->IsHTML(nsGkAtoms::progress)) {
+  if (aFrame && aFrame->GetContent()->IsHTMLElement(nsGkAtoms::progress)) {
     return static_cast<HTMLProgressElement*>(aFrame->GetContent())->Value();
   }
 
@@ -186,7 +186,7 @@ nsNativeTheme::GetProgressMaxValue(nsIFrame* aFrame)
 {
   // When we are using the HTML progress element,
   // we can get the max from the IDL property.
-  if (aFrame && aFrame->GetContent()->IsHTML(nsGkAtoms::progress)) {
+  if (aFrame && aFrame->GetContent()->IsHTMLElement(nsGkAtoms::progress)) {
     return static_cast<HTMLProgressElement*>(aFrame->GetContent())->Max();
   }
 
@@ -201,7 +201,7 @@ nsNativeTheme::GetCheckedOrSelected(nsIFrame* aFrame, bool aCheckSelected)
 
   nsIContent* content = aFrame->GetContent();
 
-  if (content->IsXUL()) {
+  if (content->IsXULElement()) {
     // For a XUL checkbox or radio button, the state of the parent determines
     // the checked state
     aFrame = aFrame->GetParent();
@@ -250,7 +250,7 @@ nsNativeTheme::GetIndeterminate(nsIFrame* aFrame)
 
   nsIContent* content = aFrame->GetContent();
 
-  if (content->IsXUL()) {
+  if (content->IsXULElement()) {
     // For a XUL checkbox or radio button, the state of the parent determines
     // the state
     return CheckBooleanAttr(aFrame->GetParent(), nsGkAtoms::indeterminate);
@@ -349,7 +349,7 @@ nsNativeTheme::IsWidgetStyled(nsPresContext* aPresContext, nsIFrame* aFrame,
           aWidgetType == NS_THEME_TEXTFIELD_MULTILINE ||
           aWidgetType == NS_THEME_LISTBOX ||
           aWidgetType == NS_THEME_DROPDOWN) &&
-         aFrame->GetContent()->IsHTML() &&
+         aFrame->GetContent()->IsHTMLElement() &&
          aPresContext->HasAuthorSpecifiedRules(aFrame,
                                                NS_AUTHOR_SPECIFIED_BORDER |
                                                NS_AUTHOR_SPECIFIED_BACKGROUND);
@@ -367,7 +367,7 @@ nsNativeTheme::IsDisabled(nsIFrame* aFrame, EventStates aEventStates)
     return false;
   }
 
-  if (content->IsHTML()) {
+  if (content->IsHTMLElement()) {
     return aEventStates.HasState(NS_EVENT_STATE_DISABLED);
   }
 
@@ -395,7 +395,7 @@ nsNativeTheme::IsHTMLContent(nsIFrame *aFrame)
     return false;
   }
   nsIContent* content = aFrame->GetContent();
-  return content && content->IsHTML();
+  return content && content->IsHTMLElement();
 }
 
 
@@ -449,12 +449,12 @@ nsNativeTheme::IsLastTreeHeaderCell(nsIFrame* aFrame)
     return false;
 
   // A tree column picker is always the last header cell.
-  if (aFrame->GetContent()->Tag() == nsGkAtoms::treecolpicker)
+  if (aFrame->GetContent()->IsXULElement(nsGkAtoms::treecolpicker))
     return true;
 
   // Find the parent tree.
   nsIContent* parent = aFrame->GetContent()->GetParent();
-  while (parent && parent->Tag() != nsGkAtoms::tree) {
+  while (parent && !parent->IsXULElement(nsGkAtoms::tree)) {
     parent = parent->GetParent();
   }
 
@@ -490,7 +490,8 @@ nsNativeTheme::IsFirstTab(nsIFrame* aFrame)
 
   nsIFrame* first = aFrame->GetParent()->GetFirstPrincipalChild();
   while (first) {
-    if (first->GetRect().width > 0 && first->GetContent()->Tag() == nsGkAtoms::tab)
+    if (first->GetRect().width > 0 &&
+        first->GetContent()->IsXULElement(nsGkAtoms::tab))
       return (first == aFrame);
     first = first->GetNextSibling();
   }
@@ -544,7 +545,7 @@ nsNativeTheme::IsIndeterminateProgress(nsIFrame* aFrame,
   if (!aFrame || !aFrame->GetContent())
     return false;
 
-  if (aFrame->GetContent()->IsHTML(nsGkAtoms::progress)) {
+  if (aFrame->GetContent()->IsHTMLElement(nsGkAtoms::progress)) {
     return aEventStates.HasState(NS_EVENT_STATE_INDETERMINATE);
   }
 
@@ -575,7 +576,7 @@ nsNativeTheme::IsSubmenu(nsIFrame* aFrame, bool* aLeftOfParent)
     return false;
 
   nsIContent* parentContent = aFrame->GetContent()->GetParent();
-  if (!parentContent || parentContent->Tag() != nsGkAtoms::menu)
+  if (!parentContent || !parentContent->IsXULElement(nsGkAtoms::menu))
     return false;
 
   nsIFrame* parent = aFrame;
diff --git a/widget/windows/nsNativeThemeWin.cpp b/widget/windows/nsNativeThemeWin.cpp
index 4168572b20..013c9da02f 100644
--- a/widget/windows/nsNativeThemeWin.cpp
+++ b/widget/windows/nsNativeThemeWin.cpp
@@ -851,7 +851,7 @@ bool
 nsNativeThemeWin::IsMenuActive(nsIFrame *aFrame, uint8_t aWidgetType)
 {
   nsIContent* content = aFrame->GetContent();
-  if (content->IsXUL() &&
+  if (content->IsXULElement() &&
       content->NodeInfo()->Equals(nsGkAtoms::richlistitem))
     return CheckBooleanAttr(aFrame, nsGkAtoms::selected);
 
@@ -971,7 +971,7 @@ nsNativeThemeWin::GetThemePartAndState(nsIFrame* aFrame, uint8_t aWidgetType,
           /* XUL textboxes don't get focused themselves, because they have child
            * html:input.. but we can check the XUL focused attributes on them
            */
-          if (content && content->IsXUL() && IsFocused(aFrame))
+          if (content && content->IsXULElement() && IsFocused(aFrame))
             aState = TFS_EDITBORDER_FOCUSED;
           else if (eventState.HasAtLeastOneOfStates(NS_EVENT_STATE_ACTIVE | NS_EVENT_STATE_FOCUS))
             aState = TFS_EDITBORDER_FOCUSED;
@@ -1290,7 +1290,7 @@ nsNativeThemeWin::GetThemePartAndState(nsIFrame* aFrame, uint8_t aWidgetType,
     }
     case NS_THEME_DROPDOWN: {
       nsIContent* content = aFrame->GetContent();
-      bool isHTML = content && content->IsHTML();
+      bool isHTML = content && content->IsHTMLElement();
       bool useDropBorder = isHTML || IsMenuListEditable(aFrame);
       EventStates eventState = GetContentState(aFrame, aWidgetType);
 
@@ -1888,7 +1888,7 @@ RENDER_AGAIN:
   // Draw focus rectangles for XP HTML checkboxes and radio buttons
   // XXX it'd be nice to draw these outside of the frame
   if (((aWidgetType == NS_THEME_CHECKBOX || aWidgetType == NS_THEME_RADIO) &&
-        aFrame->GetContent()->IsHTML()) ||
+        aFrame->GetContent()->IsHTMLElement()) ||
       aWidgetType == NS_THEME_RANGE ||
       aWidgetType == NS_THEME_SCALE_HORIZONTAL ||
       aWidgetType == NS_THEME_SCALE_VERTICAL) {
@@ -2049,7 +2049,7 @@ nsNativeThemeWin::GetWidgetBorder(nsDeviceContext* aContext,
                  aWidgetType == NS_THEME_TEXTFIELD ||
                  aWidgetType == NS_THEME_TEXTFIELD_MULTILINE)) {
     nsIContent* content = aFrame->GetContent();
-    if (content && content->IsHTML()) {
+    if (content && content->IsHTMLElement()) {
       // We need to pad textfields by 1 pixel, since the caret will draw
       // flush against the edge by default if we don't.
       aResult->top++;
@@ -2387,7 +2387,7 @@ nsNativeThemeWin::GetMinimumWidgetSize(nsPresContext* aPresContext, nsIFrame* aF
       // FIXME bug 403934: We should probably really separate
       // GetPreferredWidgetSize from GetMinimumWidgetSize, so callers can
       // use the one they want.
-      if (aFrame->GetContent()->IsHTML()) {
+      if (aFrame->GetContent()->IsHTMLElement()) {
         sizeReq = TS_MIN;
       }
       break;
@@ -3120,7 +3120,7 @@ nsresult nsNativeThemeWin::ClassicGetThemePartAndState(nsIFrame* aFrame, uint8_t
           const nsStyleUserInterface *uiData = aFrame->StyleUserInterface();
           // The down state is flat if the button is focusable
           if (uiData->mUserFocus == NS_STYLE_USER_FOCUS_NORMAL) {
-            if (!aFrame->GetContent()->IsHTML())
+            if (!aFrame->GetContent()->IsHTMLElement())
               aState |= DFCS_FLAT;
 
             aFocused = true;
@@ -3162,7 +3162,7 @@ nsresult nsNativeThemeWin::ClassicGetThemePartAndState(nsIFrame* aFrame, uint8_t
       }
 
       contentState = GetContentState(aFrame, aWidgetType);
-      if (!content->IsXUL() &&
+      if (!content->IsXULElement() &&
           contentState.HasState(NS_EVENT_STATE_FOCUS)) {
         aFocused = true;
       }
@@ -3682,7 +3682,7 @@ RENDER_AGAIN:
 
       // Fill in background
       if (IsDisabled(aFrame, eventState) ||
-          (aFrame->GetContent()->IsXUL() &&
+          (aFrame->GetContent()->IsXULElement() &&
            IsReadOnly(aFrame)))
         ::FillRect(hdc, &widgetRect, (HBRUSH) (COLOR_BTNFACE+1));
       else
diff --git a/xpfe/appshell/nsXULWindow.cpp b/xpfe/appshell/nsXULWindow.cpp
index d9592790e8..bdc2e191a0 100644
--- a/xpfe/appshell/nsXULWindow.cpp
+++ b/xpfe/appshell/nsXULWindow.cpp
@@ -1465,7 +1465,7 @@ NS_IMETHODIMP nsXULWindow::SavePersistentAttributes()
 
   // fetch docShellElement's ID and XUL owner document
   ownerXULDoc = do_QueryInterface(docShellElement->OwnerDoc());
-  if (docShellElement->IsXUL()) {
+  if (docShellElement->IsXULElement()) {
     docShellElement->GetId(windowElementId);
   }