diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index 8647825035..f8f9d5a28c 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -160,5 +160,25 @@ DocumentOrShadowRoot::GetPointerLockElement()
       retargetedPointerLockedElement->AsElement() : nullptr;
 }
 
+Element*
+DocumentOrShadowRoot::GetFullscreenElement()
+{
+  if (!AsNode().IsInComposedDoc()) {
+    return nullptr;
+  }
+
+  Element* element = AsNode().OwnerDoc()->FullScreenStackTop();
+  NS_ASSERTION(!element ||
+               element->State().HasState(NS_EVENT_STATE_FULL_SCREEN),
+    "Fullscreen element should have fullscreen styles applied");
+
+  nsIContent* retargeted = Retarget(element);
+  if (retargeted && retargeted->IsElement()) {
+    return retargeted->AsElement();
+  }
+
+  return nullptr;
+}
+
 }
 }
diff --git a/dom/base/DocumentOrShadowRoot.h b/dom/base/DocumentOrShadowRoot.h
index e622107d03..e056078275 100644
--- a/dom/base/DocumentOrShadowRoot.h
+++ b/dom/base/DocumentOrShadowRoot.h
@@ -115,6 +115,7 @@ public:
   ~DocumentOrShadowRoot() = default;
 
   Element* GetPointerLockElement();
+  Element* GetFullscreenElement();
 
 protected:
   nsIContent* Retarget(nsIContent* aContent) const;
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index a7af75979f..e158d54d76 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -8603,7 +8603,7 @@ nsDocument::OnPageHide(bool aPersisted,
   EnumerateActivityObservers(NotifyActivityChanged, nullptr);
 
   ClearPendingFullscreenRequests(this);
-  if (GetFullscreenElement()) {
+  if (FullScreenStackTop()) {
     // If this document was fullscreen, we should exit fullscreen in this
     // doctree branch. This ensures that if the user navigates while in
     // fullscreen mode we don't leave its still visible ancestor documents
@@ -10307,7 +10307,7 @@ nsIDocument::AsyncExitFullscreen(nsIDocument* aDoc)
 static bool
 CountFullscreenSubDocuments(nsIDocument* aDoc, void* aData)
 {
-  if (aDoc->GetFullscreenElement()) {
+  if (aDoc->FullScreenStackTop()) {
     uint32_t* count = static_cast<uint32_t*>(aData);
     (*count)++;
   }
@@ -10327,7 +10327,7 @@ nsDocument::IsFullscreenLeaf()
 {
   // A fullscreen leaf document is fullscreen, and has no fullscreen
   // subdocuments.
-  if (!GetFullscreenElement()) {
+  if (!FullScreenStackTop()) {
     return false;
   }
   return CountFullscreenSubDocuments(this) == 0;
@@ -10336,11 +10336,11 @@ nsDocument::IsFullscreenLeaf()
 static bool
 ResetFullScreen(nsIDocument* aDocument, void* aData)
 {
-  if (aDocument->GetFullscreenElement()) {
+  if (aDocument->FullScreenStackTop()) {
     NS_ASSERTION(CountFullscreenSubDocuments(aDocument) <= 1,
         "Should have at most 1 fullscreen subdocument.");
     static_cast<nsDocument*>(aDocument)->CleanupFullscreenState();
-    NS_ASSERTION(!aDocument->GetFullscreenElement(),
+    NS_ASSERTION(!aDocument->FullScreenStackTop(),
                  "Should reset full-screen");
     auto changed = reinterpret_cast<nsCOMArray<nsIDocument>*>(aData);
     changed->AppendElement(aDocument);
@@ -10388,7 +10388,7 @@ nsIDocument::ExitFullscreenInDocTree(nsIDocument* aMaybeNotARootDoc)
   UnlockPointer();
 
   nsCOMPtr<nsIDocument> root = aMaybeNotARootDoc->GetFullscreenRoot();
-  if (!root || !root->GetFullscreenElement()) {
+  if (!root || !root->FullScreenStackTop()) {
     // If a document was detached before exiting from fullscreen, it is
     // possible that the root had left fullscreen state. In this case,
     // we would not get anything from the ResetFullScreen() call. Root's
@@ -10417,7 +10417,7 @@ nsIDocument::ExitFullscreenInDocTree(nsIDocument* aMaybeNotARootDoc)
     DispatchFullScreenChange(changed[changed.Length() - i - 1]);
   }
 
-  NS_ASSERTION(!root->GetFullscreenElement(),
+  NS_ASSERTION(!root->FullScreenStackTop(),
     "Fullscreen root should no longer be a fullscreen doc...");
 
   // Move the top-level window out of fullscreen mode.
@@ -10434,7 +10434,7 @@ GetFullscreenLeaf(nsIDocument* aDoc, void* aData)
     nsIDocument** result = static_cast<nsIDocument**>(aData);
     *result = aDoc;
     return false;
-  } else if (aDoc->GetFullscreenElement()) {
+  } else if (aDoc->FullScreenStackTop()) {
     aDoc->EnumerateSubDocuments(GetFullscreenLeaf, aData);
   }
   return true;
@@ -10453,7 +10453,7 @@ GetFullscreenLeaf(nsIDocument* aDoc)
   nsIDocument* root = nsContentUtils::GetRootDocument(aDoc);
   // Check that the root is actually fullscreen so we don't waste time walking
   // around its descendants.
-  if (!root->GetFullscreenElement()) {
+  if (!root->FullScreenStackTop()) {
     return nullptr;
   }
   GetFullscreenLeaf(root, &leaf);
@@ -10463,10 +10463,10 @@ GetFullscreenLeaf(nsIDocument* aDoc)
 void
 nsDocument::RestorePreviousFullScreenState()
 {
-  NS_ASSERTION(!GetFullscreenElement() || !FullscreenRoots::IsEmpty(),
+  NS_ASSERTION(!FullScreenStackTop() || !FullscreenRoots::IsEmpty(),
     "Should have at least 1 fullscreen root when fullscreen!");
 
-  if (!GetFullscreenElement() || !GetWindow() || FullscreenRoots::IsEmpty()) {
+  if (!FullScreenStackTop() || !GetWindow() || FullscreenRoots::IsEmpty()) {
     return;
   }
 
@@ -10641,7 +10641,7 @@ nsDocument::FullScreenStackPush(Element* aElement)
   }
   EventStateManager::SetFullScreenState(aElement, true);
   mFullScreenStack.AppendElement(do_GetWeakReference(aElement));
-  NS_ASSERTION(GetFullscreenElement() == aElement, "Should match");
+  NS_ASSERTION(FullScreenStackTop() == aElement, "Should match");
   UpdateViewportScrollbarOverrideForFullscreen(this);
   return true;
 }
@@ -10689,7 +10689,7 @@ nsDocument::FullScreenStackTop()
   uint32_t last = mFullScreenStack.Length() - 1;
   nsCOMPtr<Element> element(do_QueryReferent(mFullScreenStack[last]));
   NS_ASSERTION(element, "Should have full-screen element!");
-  NS_ASSERTION(element->IsInUncomposedDoc(), "Full-screen element should be in doc");
+  NS_ASSERTION(element->IsInComposedDoc(), "Full-screen element should be in doc");
   NS_ASSERTION(element->OwnerDoc() == this, "Full-screen element should be in this doc");
   return element;
 }
@@ -10767,7 +10767,7 @@ nsresult nsDocument::RemoteFrameFullscreenReverted()
 }
 
 /* static */ bool
-nsDocument::IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject)
+nsIDocument::IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject)
 {
   MOZ_ASSERT(NS_IsMainThread());
   return nsContentUtils::IsCallerChrome() ||
@@ -10815,10 +10815,10 @@ nsDocument::FullscreenElementReadyCheck(Element* aElement,
 {
   NS_ASSERTION(aElement,
     "Must pass non-null element to nsDocument::RequestFullScreen");
-  if (!aElement || aElement == GetFullscreenElement()) {
+  if (!aElement || aElement == FullScreenStackTop()) {
     return false;
   }
-  if (!aElement->IsInUncomposedDoc()) {
+  if (!aElement->IsInComposedDoc()) {
     DispatchFullscreenError("FullscreenDeniedNotInDocument");
     return false;
   }
@@ -10842,8 +10842,11 @@ nsDocument::FullscreenElementReadyCheck(Element* aElement,
     DispatchFullscreenError("FullscreenDeniedSubDocFullScreen");
     return false;
   }
-  if (GetFullscreenElement() &&
-      !nsContentUtils::ContentIsDescendantOf(aElement, GetFullscreenElement())) {
+  //XXXsmaug Note, we don't follow the latest fullscreen spec here.
+  //         This whole check could be probably removed.
+  if (FullScreenStackTop() &&
+      !nsContentUtils::ContentIsHostIncludingDescendantOf(aElement,
+                                                          FullScreenStackTop())) {
     // If this document is full-screen, only grant full-screen requests from
     // a descendant of the current full-screen element.
     DispatchFullscreenError("FullscreenDeniedNotDescendant");
@@ -11203,16 +11206,6 @@ nsDocument::GetMozFullScreenElement(nsIDOMElement **aFullScreenElement)
   return NS_OK;
 }
 
-Element*
-nsDocument::GetFullscreenElement()
-{
-  Element* element = FullScreenStackTop();
-  NS_ASSERTION(!element ||
-               element->State().HasState(NS_EVENT_STATE_FULL_SCREEN),
-    "Fullscreen element should have fullscreen styles applied");
-  return element;
-}
-
 NS_IMETHODIMP
 nsDocument::GetMozFullScreen(bool *aFullScreen)
 {
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 010f95ae25..1c10d7b5c3 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -896,8 +896,6 @@ public:
   //
   already_AddRefed<nsSimpleContentList> BlockedTrackingNodes() const;
 
-  static bool IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject);
-
   // Do the "fullscreen element ready check" from the fullscreen spec.
   // It returns true if the given element is allowed to go into fullscreen.
   bool FullscreenElementReadyCheck(Element* aElement, bool aWasCallerChrome);
@@ -921,11 +919,10 @@ public:
   void FullScreenStackPop();
 
   // Returns the top element from the full-screen stack.
-  Element* FullScreenStackTop();
+  Element* FullScreenStackTop() override;
 
   // DOM-exposed fullscreen API
   bool FullscreenEnabled() override;
-  Element* GetFullscreenElement() override;
 
   void RequestPointerLock(Element* aElement) override;
   bool SetPointerLock(Element* aElement, int aCursorStyle);
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 3bdbd0896c..dcaee4fabd 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -2637,7 +2637,7 @@ public:
   nsIURI* GetDocumentURIObject() const;
   // Not const because all the full-screen goop is not const
   virtual bool FullscreenEnabled() = 0;
-  virtual Element* GetFullscreenElement() = 0;
+  virtual Element* FullScreenStackTop() = 0;
   bool Fullscreen()
   {
     return !!GetFullscreenElement();
@@ -2647,6 +2647,7 @@ public:
   {
     UnlockPointer(this);
   }
+  static bool IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject);
 #ifdef MOZILLA_INTERNAL_API
   bool Hidden() const
   {
diff --git a/dom/html/test/file_fullscreen-shadowdom.html b/dom/html/test/file_fullscreen-shadowdom.html
new file mode 100644
index 0000000000..efc7fe8f5f
--- /dev/null
+++ b/dom/html/test/file_fullscreen-shadowdom.html
@@ -0,0 +1,52 @@
+<!DOCTYPE HTML>
+<html>
+  <!--
+  https://bugzilla.mozilla.org/show_bug.cgi?id=1430305
+  Bug 1430305 - Implement ShadowRoot.fullscreenElement
+  -->
+  <head>
+    <title>Bug 1430305</title>
+    <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js">
+    </script>
+    <script type="application/javascript" src="/tests/SimpleTest/EventUtils.js">
+    </script>
+    <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+  </head>
+  <body>
+    <a target="_blank"
+       href="https://bugzilla.mozilla.org/show_bug.cgi?id=1430305">
+      Mozilla Bug 1430305</a>
+
+    <div id="host"></div>
+
+    <pre id="test">
+      <script type="application/javascript">
+
+        function begin() {
+          var host = document.getElementById("host");
+          var shadowRoot = host.attachShadow({mode: "open"});
+          shadowRoot.innerHTML = "<div>test</div>";
+          var elem = shadowRoot.firstChild;
+          var gotFullscreenEvent = false;
+
+          document.addEventListener("fullscreenchange", function (e) {
+            if (document.fullscreenElement === host) {
+              is(shadowRoot.fullscreenElement, elem,
+                 "Expected element entered fullsceen");
+              gotFullscreenEvent = true;
+              document.exitFullscreen();
+            } else {
+              opener.ok(gotFullscreenEvent, "Entered fullscreen as expected");
+              is(shadowRoot.fullscreenElement, null,
+                 "Shouldn't have fullscreenElement anymore.");
+              is(document.fullscreenElement, null,
+                 "Shouldn't have fullscreenElement anymore.");
+              opener.nextTest();
+            }
+          });
+          elem.requestFullscreen();
+        }
+      </script>
+    </pre>
+  </body>
+</html>
diff --git a/dom/html/test/mochitest.ini b/dom/html/test/mochitest.ini
index dcbb73840a..a0cc7b77f8 100644
--- a/dom/html/test/mochitest.ini
+++ b/dom/html/test/mochitest.ini
@@ -72,6 +72,7 @@ support-files =
   file_fullscreen-rollback.html
   file_fullscreen-scrollbar.html
   file_fullscreen-selector.html
+  file_fullscreen-shadowdom.html
   file_fullscreen-svg-element.html
   file_fullscreen-top-layer.html
   file_fullscreen-unprefix-disabled-inner.html
diff --git a/dom/html/test/test_fullscreen-api.html b/dom/html/test/test_fullscreen-api.html
index 1ac5b3b517..e269c67ad4 100644
--- a/dom/html/test/test_fullscreen-api.html
+++ b/dom/html/test/test_fullscreen-api.html
@@ -39,6 +39,7 @@ var gTestWindows = [
   "file_fullscreen-navigation.html",
   "file_fullscreen-scrollbar.html",
   "file_fullscreen-selector.html",
+  "file_fullscreen-shadowdom.html",
   "file_fullscreen-top-layer.html",
   "file_fullscreen-backdrop.html",
   "file_fullscreen-nested.html",
@@ -96,7 +97,8 @@ addLoadEvent(function() {
         ["full-screen-api.unprefix.enabled", true],
         ["full-screen-api.allow-trusted-requests-only", false],
         ["full-screen-api.transition-duration.enter", "0 0"],
-        ["full-screen-api.transition-duration.leave", "0 0"]
+        ["full-screen-api.transition-duration.leave", "0 0"],
+        ["dom.webcomponents.enabled", true]
       ]}, nextTest);
 });
 SimpleTest.waitForExplicitFinish();
diff --git a/dom/webidl/Document.webidl b/dom/webidl/Document.webidl
index 46cb629feb..3bdd4e7962 100644
--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -238,10 +238,6 @@ partial interface Document {
   readonly attribute boolean fullscreenEnabled;
   [BinaryName="fullscreenEnabled"]
   readonly attribute boolean mozFullScreenEnabled;
-  [LenientSetter, Func="nsDocument::IsUnprefixedFullscreenEnabled"]
-  readonly attribute Element? fullscreenElement;
-  [BinaryName="fullscreenElement"]
-  readonly attribute Element? mozFullScreenElement;
 
   [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
   void exitFullscreen();
diff --git a/dom/webidl/DocumentOrShadowRoot.webidl b/dom/webidl/DocumentOrShadowRoot.webidl
index 267f16f23b..eb4a4b666d 100644
--- a/dom/webidl/DocumentOrShadowRoot.webidl
+++ b/dom/webidl/DocumentOrShadowRoot.webidl
@@ -23,6 +23,8 @@ interface DocumentOrShadowRoot {
   readonly attribute StyleSheetList styleSheets;
 
   readonly attribute Element? pointerLockElement;
-  // Not implemented yet: bug 1430305.
-  // readonly attribute Element? fullscreenElement;
+  [LenientSetter, Func="nsDocument::IsUnprefixedFullscreenEnabled"]
+  readonly attribute Element? fullscreenElement;
+  [BinaryName="fullscreenElement"]
+  readonly attribute Element? mozFullScreenElement;
 };