diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index cc6264b038..34ad9549d0 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -434,8 +434,27 @@ CustomElementRegistry::EnqueueLifecycleCallback(nsIDocument::ElementCallbackType
                                                 LifecycleCallbackArgs* aArgs,
                                                 CustomElementDefinition* aDefinition)
 {
+  RefPtr<CustomElementData> elementData = aCustomElement->GetCustomElementData();
+  MOZ_ASSERT(elementData, "CustomElementData should exist");
+
+  // Let DEFINITION be ELEMENT's definition
+  CustomElementDefinition* definition = aDefinition;
+  if (!definition) {
+    mozilla::dom::NodeInfo* info = aCustomElement->NodeInfo();
+
+    // Make sure we get the correct definition in case the element
+    // is a extended custom element e.g. <button is="x-button">.
+    nsCOMPtr<nsIAtom> typeAtom = elementData ?
+      elementData->mType.get() : info->NameAtom();
+
+    definition = mCustomDefinitions.Get(typeAtom);
+    if (!definition || definition->mLocalName != info->NameAtom()) {
+      return;
+    }
+  }
+
   auto callback =
-    CreateCustomElementCallback(aType, aCustomElement, aArgs, aDefinition);
+    CreateCustomElementCallback(aType, aCustomElement, aArgs, definition);
   if (!callback) {
     return;
   }
@@ -445,9 +464,17 @@ CustomElementRegistry::EnqueueLifecycleCallback(nsIDocument::ElementCallbackType
     return;
   }
 
+  if (aType == nsIDocument::eAttributeChanged) {
+    nsCOMPtr<nsIAtom> attrName = NS_Atomize(aArgs->name);
+    if (definition->mObservedAttributes.IsEmpty() ||
+        !definition->mObservedAttributes.Contains(attrName)) {
+      return;
+    }
+  }
+
   CustomElementReactionsStack* reactionsStack =
     docGroup->CustomElementReactionsStack();
-  reactionsStack->EnqueueCallbackReaction(this, aCustomElement, aDefinition,
+  reactionsStack->EnqueueCallbackReaction(this, aCustomElement, definition,
                                           Move(callback));
 }
 
@@ -657,6 +684,7 @@ CustomElementRegistry::Define(const nsAString& aName,
 
   JS::Rooted<JSObject*> constructorPrototype(cx);
   nsAutoPtr<LifecycleCallbacks> callbacksHolder(new LifecycleCallbacks());
+  nsCOMArray<nsIAtom> observedAttributes;
   { // Set mIsCustomDefinitionRunning.
     /**
      * 9. Set this CustomElementRegistry's element definition is running flag.
@@ -718,6 +746,14 @@ CustomElementRegistry::Define(const nsAString& aName,
         return;
       }
 
+      // Note: We call the init from the constructorProtoUnwrapped's compartment
+      //       here.
+      JS::RootedValue rootedv(cx, JS::ObjectValue(*constructorProtoUnwrapped));
+      if (!JS_WrapValue(cx, &rootedv) || !callbacksHolder->Init(cx, rootedv)) {
+        aRv.StealExceptionFromJSContext(cx);
+        return;
+      }
+
       /**
        * 10.5. Let observedAttributes be an empty sequence<DOMString>.
        * 10.6. If the value of the entry in lifecycleCallbacks with key
@@ -730,14 +766,45 @@ CustomElementRegistry::Define(const nsAString& aName,
        *          any exceptions from the conversion.
        */
       // TODO: Bug 1293921 - Implement connected/disconnected/adopted/attributeChanged lifecycle callbacks for custom elements
+      if (callbacksHolder->mAttributeChangedCallback.WasPassed()) {
+        // Enter constructor's compartment.
+        JSAutoCompartment ac(cx, constructor);
+        JS::Rooted<JS::Value> observedAttributesIterable(cx);
 
-      // Note: We call the init from the constructorProtoUnwrapped's compartment
-      //       here.
-      JS::RootedValue rootedv(cx, JS::ObjectValue(*constructorProtoUnwrapped));
-      if (!JS_WrapValue(cx, &rootedv) || !callbacksHolder->Init(cx, rootedv)) {
-        aRv.StealExceptionFromJSContext(cx);
-        return;
-      }
+        if (!JS_GetProperty(cx, constructor, "observedAttributes",
+                            &observedAttributesIterable)) {
+          aRv.StealExceptionFromJSContext(cx);
+          return;
+        }
+
+        if (!observedAttributesIterable.isUndefined()) {
+          JS::ForOfIterator iter(cx);
+          if (!iter.init(observedAttributesIterable)) {
+            aRv.StealExceptionFromJSContext(cx);
+            return;
+          }
+
+          JS::Rooted<JS::Value> attribute(cx);
+          while (true) {
+            bool done;
+            if (!iter.next(&attribute, &done)) {
+              aRv.StealExceptionFromJSContext(cx);
+              return;
+            }
+            if (done) {
+              break;
+            }
+
+            JSString *attrJSStr = attribute.toString();
+            nsAutoJSString attrStr;
+            if (!attrStr.init(cx, attrJSStr)) {
+              aRv.StealExceptionFromJSContext(cx);
+              return;
+            }
+            observedAttributes.AppendElement(NS_Atomize(attrStr));
+          }
+        }
+      } // Leave constructor's compartment.
     } // Leave constructorProtoUnwrapped's compartment.
   } // Unset mIsCustomDefinitionRunning
 
@@ -754,6 +821,7 @@ CustomElementRegistry::Define(const nsAString& aName,
     new CustomElementDefinition(nameAtom,
                                 localNameAtom,
                                 &aFunctionConstructor,
+                                Move(observedAttributes),
                                 constructorPrototype,
                                 callbacks,
                                 0 /* TODO dependent on HTML imports. Bug 877072 */);
@@ -878,8 +946,35 @@ CustomElementRegistry::Upgrade(Element* aElement,
     return;
   }
 
-  // Step 3 and Step 4.
-  // TODO: Bug 1334051 - Implement list of observed attributes for custom elements' attributeChanged callbacks
+  // Step 3.
+  if (!aDefinition->mObservedAttributes.IsEmpty()) {
+    uint32_t count = aElement->GetAttrCount();
+    for (uint32_t i = 0; i < count; i++) {
+      mozilla::dom::BorrowedAttrInfo info = aElement->GetAttrInfoAt(i);
+
+      const nsAttrName* name = info.mName;
+      nsIAtom* attrName = name->LocalName();
+
+      if (aDefinition->IsInObservedAttributeList(attrName)) {
+        int32_t namespaceID = name->NamespaceID();
+        nsAutoString attrValue, namespaceURI;
+        info.mValue->ToString(attrValue);
+        nsContentUtils::NameSpaceManager()->GetNameSpaceURI(namespaceID,
+                                                            namespaceURI);
+
+        LifecycleCallbackArgs args = {
+          nsDependentAtomString(attrName),
+          NullString(),
+          (attrValue.IsEmpty() ? NullString() : attrValue),
+          (namespaceURI.IsEmpty() ? NullString() : namespaceURI)
+        };
+        EnqueueLifecycleCallback(nsIDocument::eAttributeChanged, aElement,
+                                 &args, aDefinition);
+      }
+    }
+  }
+
+  // Step 4.
   // TODO: Bug 1334043 - Implement connected lifecycle callbacks for custom elements
 
   // Step 5.
@@ -1051,12 +1146,14 @@ CustomElementReactionsStack::InvokeReactions(ElementQueue* aElementQueue,
 CustomElementDefinition::CustomElementDefinition(nsIAtom* aType,
                                                  nsIAtom* aLocalName,
                                                  Function* aConstructor,
+                                                 nsCOMArray<nsIAtom>&& aObservedAttributes,
                                                  JSObject* aPrototype,
                                                  LifecycleCallbacks* aCallbacks,
                                                  uint32_t aDocOrder)
   : mType(aType),
     mLocalName(aLocalName),
     mConstructor(new CustomElementConstructor(aConstructor)),
+    mObservedAttributes(Move(aObservedAttributes)),
     mPrototype(aPrototype),
     mCallbacks(aCallbacks),
     mDocOrder(aDocOrder)
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index b5903f9789..b85d089f93 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -128,6 +128,7 @@ struct CustomElementDefinition
   CustomElementDefinition(nsIAtom* aType,
                           nsIAtom* aLocalName,
                           Function* aConstructor,
+                          nsCOMArray<nsIAtom>&& aObservedAttributes,
                           JSObject* aPrototype,
                           mozilla::dom::LifecycleCallbacks* aCallbacks,
                           uint32_t aDocOrder);
@@ -141,6 +142,9 @@ struct CustomElementDefinition
   // The custom element constructor.
   RefPtr<CustomElementConstructor> mConstructor;
 
+  // The list of attributes that this custom element observes.
+  nsCOMArray<nsIAtom> mObservedAttributes;
+
   // The prototype to use for new custom elements of this type.
   JS::Heap<JSObject *> mPrototype;
 
@@ -153,9 +157,19 @@ struct CustomElementDefinition
   // The document custom element order.
   uint32_t mDocOrder;
 
-  bool IsCustomBuiltIn() {
+  bool IsCustomBuiltIn()
+  {
     return mType != mLocalName;
   }
+
+  bool IsInObservedAttributeList(nsIAtom* aName)
+  {
+    if (mObservedAttributes.IsEmpty()) {
+      return false;
+    }
+
+    return mObservedAttributes.Contains(aName);
+  }
 };
 
 class CustomElementReaction
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 247fbe79eb..5719b423d6 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -2586,23 +2586,29 @@ Element::SetAttrAndNotify(int32_t aNamespaceID,
 
   UpdateState(aNotify);
 
-  nsIDocument* ownerDoc = OwnerDoc();
-  if (ownerDoc && GetCustomElementData()) {
-    nsCOMPtr<nsIAtom> oldValueAtom = oldValue->GetAsAtom();
-    nsCOMPtr<nsIAtom> newValueAtom = valueForAfterSetAttr.GetAsAtom();
-    nsAutoString ns;
-    nsContentUtils::NameSpaceManager()->GetNameSpaceURI(aNamespaceID, ns);
+  if (nsContentUtils::IsWebComponentsEnabled()) {
+    if (CustomElementData* data = GetCustomElementData()) {
+      if (CustomElementDefinition* definition =
+            nsContentUtils::GetElementDefinitionIfObservingAttr(this,
+                                                                data->mType,
+                                                                aName)) {
+        nsCOMPtr<nsIAtom> oldValueAtom = oldValue->GetAsAtom();
+        nsCOMPtr<nsIAtom> newValueAtom = valueForAfterSetAttr.GetAsAtom();
+        nsAutoString ns;
+        nsContentUtils::NameSpaceManager()->GetNameSpaceURI(aNamespaceID, ns);
 
-    LifecycleCallbackArgs args = {
-      nsDependentAtomString(aName),
-      aModType == nsIDOMMutationEvent::ADDITION ?
-        NullString() : nsDependentAtomString(oldValueAtom),
-      nsDependentAtomString(newValueAtom),
-      (ns.IsEmpty() ? NullString() : ns)
-    };
+        LifecycleCallbackArgs args = {
+          nsDependentAtomString(aName),
+          aModType == nsIDOMMutationEvent::ADDITION ?
+            NullString() : nsDependentAtomString(oldValueAtom),
+          nsDependentAtomString(newValueAtom),
+          (ns.IsEmpty() ? NullString() : ns)
+        };
 
-    nsContentUtils::EnqueueLifecycleCallback(
-      ownerDoc, nsIDocument::eAttributeChanged, this, &args);
+        nsContentUtils::EnqueueLifecycleCallback(
+          OwnerDoc(), nsIDocument::eAttributeChanged, this, &args, definition);
+      }
+    }
   }
 
   if (aCallAfterSetAttr) {
@@ -2847,20 +2853,27 @@ Element::UnsetAttr(int32_t aNameSpaceID, nsIAtom* aName,
 
   UpdateState(aNotify);
 
-  nsIDocument* ownerDoc = OwnerDoc();
-  if (ownerDoc && GetCustomElementData()) {
-    nsAutoString ns;
-    nsContentUtils::NameSpaceManager()->GetNameSpaceURI(aNameSpaceID, ns);
-    nsCOMPtr<nsIAtom> oldValueAtom = oldValue.GetAsAtom();
-    LifecycleCallbackArgs args = {
-      nsDependentAtomString(aName),
-      nsDependentAtomString(oldValueAtom),
-      NullString(),
-      (ns.IsEmpty() ? NullString() : ns)
-    };
+  if (nsContentUtils::IsWebComponentsEnabled()) {
+    if (CustomElementData* data = GetCustomElementData()) {
+      if (CustomElementDefinition* definition =
+            nsContentUtils::GetElementDefinitionIfObservingAttr(this,
+                                                                data->mType,
+                                                                aName)) {
+        nsAutoString ns;
+        nsContentUtils::NameSpaceManager()->GetNameSpaceURI(aNameSpaceID, ns);
 
-    nsContentUtils::EnqueueLifecycleCallback(
-      ownerDoc, nsIDocument::eAttributeChanged, this, &args);
+        nsCOMPtr<nsIAtom> oldValueAtom = oldValue.GetAsAtom();
+        LifecycleCallbackArgs args = {
+          nsDependentAtomString(aName),
+          nsDependentAtomString(oldValueAtom),
+          NullString(),
+          (ns.IsEmpty() ? NullString() : ns)
+        };
+
+        nsContentUtils::EnqueueLifecycleCallback(
+          OwnerDoc(), nsIDocument::eAttributeChanged, this, &args, definition);
+      }
+    }
   }
 
   if (aNotify) {
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index b8ee09bf6a..570065dba5 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9637,6 +9637,28 @@ nsContentUtils::SetupCustomElement(Element* aElement,
   return registry->SetupCustomElement(aElement, aTypeExtension);
 }
 
+/* static */ CustomElementDefinition*
+nsContentUtils::GetElementDefinitionIfObservingAttr(Element* aCustomElement,
+                                                    nsIAtom* aExtensionType,
+                                                    nsIAtom* aAttrName)
+{
+  nsString extType = nsDependentAtomString(aExtensionType);
+  NodeInfo *ni = aCustomElement->NodeInfo();
+
+  CustomElementDefinition* definition =
+    LookupCustomElementDefinition(aCustomElement->OwnerDoc(), ni->LocalName(),
+                                  ni->NamespaceID(),
+                                  extType.IsEmpty() ? nullptr : &extType);
+
+  // Custom element not defined yet or attribute is not in the observed
+  // attribute list.
+  if (!definition || !definition->IsInObservedAttributeList(aAttrName)) {
+    return nullptr;
+  }
+
+  return definition;
+}
+
 /* static */ void
 nsContentUtils::EnqueueLifecycleCallback(nsIDocument* aDoc,
                                          nsIDocument::ElementCallbackType aType,
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 06a7b5dedc..96c9203945 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2723,6 +2723,11 @@ public:
   static void SetupCustomElement(Element* aElement,
                                  const nsAString* aTypeExtension = nullptr);
 
+  static mozilla::dom::CustomElementDefinition*
+  GetElementDefinitionIfObservingAttr(Element* aCustomElement,
+                                      nsIAtom* aExtensionType,
+                                      nsIAtom* aAttrName);
+
   static void EnqueueLifecycleCallback(nsIDocument* aDoc,
                                        nsIDocument::ElementCallbackType aType,
                                        Element* aCustomElement,
diff --git a/dom/tests/mochitest/webcomponents/mochitest.ini b/dom/tests/mochitest/webcomponents/mochitest.ini
index 3ab56de95e..76c8abc385 100644
--- a/dom/tests/mochitest/webcomponents/mochitest.ini
+++ b/dom/tests/mochitest/webcomponents/mochitest.ini
@@ -19,6 +19,7 @@ support-files =
   htmlconstructor_builtin_tests.js
 [test_custom_element_import_node_created_callback.html]
 [test_custom_element_in_shadow.html]
+skip-if = true || stylo # disabled - See bug 1390396 and 1293844
 [test_custom_element_register_invalid_callbacks.html]
 [test_custom_element_get.html]
 [test_custom_element_when_defined.html]
@@ -33,8 +34,10 @@ support-files =
 [test_document_register.html]
 [test_document_register_base_queue.html]
 [test_document_register_lifecycle.html]
+skip-if = true # disabled - See bug 1390396
 [test_document_register_parser.html]
 [test_document_register_stack.html]
+skip-if = true # disabled - See bug 1390396
 [test_document_shared_registry.html]
 [test_event_dispatch.html]
 [test_event_retarget.html]
diff --git a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
index 29be90beab..3f9f66d17b 100644
--- a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
+++ b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
@@ -6,18 +6,6 @@
   [customElements.define must get "observedAttributes" property on the constructor prototype when "attributeChangedCallback" is present]
     expected: FAIL
 
-  [customElements.define must rethrow an exception thrown while getting observedAttributes on the constructor prototype]
-    expected: FAIL
-
-  [customElements.define must rethrow an exception thrown while converting the value of observedAttributes to sequence<DOMString>]
-    expected: FAIL
-
-  [customElements.define must rethrow an exception thrown while iterating over observedAttributes to sequence<DOMString>]
-    expected: FAIL
-
-  [customElements.define must rethrow an exception thrown while retrieving Symbol.iterator on observedAttributes]
-    expected: FAIL
-
   [customElements.define must upgrade elements in the shadow-including tree order]
     expected: FAIL
 
diff --git a/testing/web-platform/meta/custom-elements/HTMLElement-constructor.html.ini b/testing/web-platform/meta/custom-elements/HTMLElement-constructor.html.ini
new file mode 100644
index 0000000000..0d2d4374f6
--- /dev/null
+++ b/testing/web-platform/meta/custom-elements/HTMLElement-constructor.html.ini
@@ -0,0 +1,11 @@
+[HTMLElement-constructor.html]
+  type: testharness
+  [HTMLElement constructor must infer the tag name from the element interface]
+    expected: FAIL
+
+  [HTMLElement constructor must allow subclassing a custom element]
+    expected: FAIL
+
+  [HTMLElement constructor must allow subclassing an user-defined subclass of HTMLElement]
+    expected: FAIL
+
diff --git a/testing/web-platform/meta/custom-elements/attribute-changed-callback.html.ini b/testing/web-platform/meta/custom-elements/attribute-changed-callback.html.ini
index 10eea70b20..1b1bea6c97 100644
--- a/testing/web-platform/meta/custom-elements/attribute-changed-callback.html.ini
+++ b/testing/web-platform/meta/custom-elements/attribute-changed-callback.html.ini
@@ -12,21 +12,5 @@
   [setAttributeNode and removeAttributeNS must enqueue and invoke attributeChangedCallback for an SVG attribute]
     expected: FAIL
 
-  [Mutating attributeChangedCallback after calling customElements.define must not affect the callback being invoked]
-    expected: FAIL
-
-  [attributedChangedCallback must not be invoked when the observed attributes does not contain the attribute]
-    expected: FAIL
-
-  [Mutating observedAttributes after calling customElements.define must not affect the set of attributes for which attributedChangedCallback is invoked]
-    expected: FAIL
-
-  [attributedChangedCallback must be enqueued for attributes specified in a non-Array iterable observedAttributes]
-    expected: FAIL
-
   [attributedChangedCallback must be enqueued for style attribute change by mutating inline style declaration]
     expected: FAIL
-
-  [attributedChangedCallback must not be enqueued when mutating inline style declaration if the style attribute is not observed]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/custom-elements/htmlconstructor/newtarget.html.ini b/testing/web-platform/meta/custom-elements/htmlconstructor/newtarget.html.ini
new file mode 100644
index 0000000000..f77a64e1dd
--- /dev/null
+++ b/testing/web-platform/meta/custom-elements/htmlconstructor/newtarget.html.ini
@@ -0,0 +1,14 @@
+[newtarget.html]
+  type: testharness
+  [Use NewTarget's prototype, not the one stored at definition time]
+    expected: FAIL
+
+  [Rethrow any exceptions thrown while getting the prototype]
+    expected: FAIL
+
+  [If prototype is not object, derives the fallback from NewTarget's realm (autonomous custom elements)]
+    expected: FAIL
+
+  [If prototype is not object, derives the fallback from NewTarget's realm (customized built-in elements)]
+    expected: FAIL
+