--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -98,17 +98,16 @@ CustomElementData::RunCallbackQueue()
   mCurrentCallback = -1;
 }
 
 // Only needed for refcounted objects.
 NS_IMPL_CYCLE_COLLECTION_CLASS(CustomElementRegistry)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(CustomElementRegistry)
   tmp->mCustomDefinitions.Clear();
-  tmp->mConstructors.clear();
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mWhenDefinedPromiseMap)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mWindow)
   NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(CustomElementRegistry)
   for (auto iter = tmp->mCustomDefinitions.Iter(); !iter.Done(); iter.Next()) {
     nsAutoPtr<LifecycleCallbacks>& callbacks = iter.UserData()->mCallbacks;
@@ -146,22 +145,16 @@ NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(Cus
   for (auto iter = tmp->mCustomDefinitions.Iter(); !iter.Done(); iter.Next()) {
     aCallbacks.Trace(&iter.UserData()->mConstructor,
                      "mCustomDefinitions constructor",
                      aClosure);
     aCallbacks.Trace(&iter.UserData()->mPrototype,
                      "mCustomDefinitions prototype",
                      aClosure);
   }
-
-  for (ConstructorMap::Enum iter(tmp->mConstructors); !iter.empty(); iter.popFront()) {
-    aCallbacks.Trace(&iter.front().mutableKey(),
-                     "mConstructors key",
-                     aClosure);
-  }
   NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
 NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(CustomElementRegistry)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(CustomElementRegistry)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CustomElementRegistry)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
@@ -192,21 +185,16 @@ CustomElementRegistry::Create(nsPIDOMWin
 
   if (!Preferences::GetBool("dom.webcomponents.customelements.enabled") &&
       !Preferences::GetBool("dom.webcomponents.enabled")) {
     return nullptr;
   }
 
   RefPtr<CustomElementRegistry> customElementRegistry =
     new CustomElementRegistry(aWindow);
-
-  if (!customElementRegistry->Init()) {
-    return nullptr;
-  }
-
   return customElementRegistry.forget();
 }
 
 /* static */ void
 CustomElementRegistry::ProcessTopElementQueue()
 {
   MOZ_ASSERT(nsContentUtils::IsSafeToRunScript());
 
@@ -255,22 +243,16 @@ CustomElementRegistry::CustomElementRegi
   }
 }
 
 CustomElementRegistry::~CustomElementRegistry()
 {
   mozilla::DropJSObjects(this);
 }
 
-bool
-CustomElementRegistry::Init()
-{
-  return mConstructors.init();
-}
-
 CustomElementDefinition*
 CustomElementRegistry::LookupCustomElementDefinition(const nsAString& aLocalName,
                                                      const nsAString* aIs) const
 {
   nsCOMPtr<nsIAtom> localNameAtom = NS_Atomize(aLocalName);
   nsCOMPtr<nsIAtom> typeAtom = aIs ? NS_Atomize(*aIs) : localNameAtom;
 
   CustomElementDefinition* data = mCustomDefinitions.Get(typeAtom);
@@ -629,23 +611,19 @@ CustomElementRegistry::Define(const nsAS
     aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
     return;
   }
 
   /**
    * 4. If this CustomElementRegistry contains an entry with constructor constructor,
    *    then throw a "NotSupportedError" DOMException and abort these steps.
    */
-  const auto& ptr = mConstructors.lookup(constructorUnwrapped);
-  if (ptr) {
-    MOZ_ASSERT(mCustomDefinitions.Get(ptr->value()),
-               "Definition must be found in mCustomDefinitions");
-    aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
-    return;
-  }
+  // TODO: Step 3 of HTMLConstructor also needs a way to look up definition by
+  // using constructor. So I plans to figure out a solution to support both of
+  // them in bug 1274159.
 
   /**
    * 5. Let localName be name.
    * 6. Let extends be the value of the extends member of options, or null if
    *    no such member exists.
    * 7. If extends is not null, then:
    *    1. If extends is a valid custom element name, then throw a
    *       "NotSupportedError" DOMException.
@@ -791,26 +769,18 @@ CustomElementRegistry::Define(const nsAS
                                 constructor,
                                 constructorPrototype,
                                 callbacks,
                                 0 /* TODO dependent on HTML imports. Bug 877072 */);
 
   /**
    * 12. Add definition to this CustomElementRegistry.
    */
-  if (!mConstructors.put(constructorUnwrapped, nameAtom)) {
-    aRv.Throw(NS_ERROR_FAILURE);
-    return;
-  }
-
   mCustomDefinitions.Put(nameAtom, definition);
 
-  MOZ_ASSERT(mCustomDefinitions.Count() == mConstructors.count(),
-             "Number of entries should be the same");
-
   /**
    * 13. 14. 15. Upgrade candidates
    */
   // TODO: Bug 1299363 - Implement custom element v1 upgrade algorithm
   UpgradeCandidates(cx, nameAtom, definition);
 
   /**
    * 16. If this CustomElementRegistry's when-defined promise map contains an
@@ -885,9 +855,9 @@ CustomElementDefinition::CustomElementDe
     mConstructor(aConstructor),
     mPrototype(aPrototype),
     mCallbacks(aCallbacks),
     mDocOrder(aDocOrder)
 {
 }
 
 } // namespace dom
-} // namespace mozilla
+} // namespace mozilla
\ No newline at end of file

--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -2,24 +2,23 @@
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef mozilla_dom_CustomElementRegistry_h
 #define mozilla_dom_CustomElementRegistry_h
 
-#include "js/GCHashTable.h"
 #include "js/TypeDecls.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BindingDeclarations.h"
-#include "mozilla/dom/FunctionBinding.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsWrapperCache.h"
+#include "mozilla/dom/FunctionBinding.h"
 
 class nsDocument;
 
 namespace mozilla {
 namespace dom {
 
 struct CustomElementData;
 struct ElementDefinitionOptions;
@@ -168,18 +167,16 @@ public:
 
   void GetCustomPrototype(nsIAtom* aAtom,
                           JS::MutableHandle<JSObject*> aPrototype);
 
 private:
   explicit CustomElementRegistry(nsPIDOMWindowInner* aWindow);
   ~CustomElementRegistry();
 
-  bool Init();
-
   /**
    * Registers an unresolved custom element that is a candidate for
    * upgrade when the definition is registered via registerElement.
    * |aTypeName| is the name of the custom element type, if it is not
    * provided, then element name is used. |aTypeName| should be provided
    * when registering a custom element that extends an existing
    * element. e.g. <button is="x-button">.
    */
@@ -189,35 +186,25 @@ private:
   void UpgradeCandidates(JSContext* aCx,
                          nsIAtom* aKey,
                          CustomElementDefinition* aDefinition);
 
   typedef nsClassHashtable<nsISupportsHashKey, CustomElementDefinition>
     DefinitionMap;
   typedef nsClassHashtable<nsISupportsHashKey, nsTArray<nsWeakPtr>>
     CandidateMap;
-  typedef JS::GCHashMap<JS::Heap<JSObject*>,
-                        nsCOMPtr<nsIAtom>,
-                        js::MovableCellHasher<JS::Heap<JSObject*>>,
-                        js::SystemAllocPolicy> ConstructorMap;
 
   // Hashtable for custom element definitions in web components.
   // Custom prototypes are stored in the compartment where
   // registerElement was called.
   DefinitionMap mCustomDefinitions;
 
-  // Hashtable for looking up definitions by using constructor as key.
-  // Custom elements' name are stored here and we need to lookup
-  // mCustomDefinitions again to get definitions.
-  ConstructorMap mConstructors;
-
   typedef nsRefPtrHashtable<nsISupportsHashKey, Promise>
     WhenDefinedPromiseMap;
   WhenDefinedPromiseMap mWhenDefinedPromiseMap;
-
   // The "upgrade candidates map" from the web components spec. Maps from a
   // namespace id and local name to a list of elements to upgrade if that
   // element is registered as a custom element.
   CandidateMap mCandidatesMap;
 
   nsCOMPtr<nsPIDOMWindowInner> mWindow;
 
   // Array representing the processing stack in the custom elements

--- a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
+++ b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
@@ -1,10 +1,13 @@
 [CustomElementRegistry.html]
   type: testharness
+  [customElements.define must throw a NotSupportedError when there is already a custom element with the same class]
+    expected: FAIL
+
   [customElements.define must get callbacks of the constructor prototype]
     expected: FAIL
 
   [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

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/meta/custom-elements/custom-element-registry/define.html.ini
@@ -0,0 +1,26 @@
+[define.html]
+  type: testharness
+  [If constructor is HTMLElement, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is HTMLButtonElement, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is HTMLImageElement, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is HTMLMediaElement, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is Image, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is Audio, should throw a TypeError]
+    expected: FAIL
+
+  [If constructor is Option, should throw a TypeError]
+    expected: FAIL
+
+  [If the constructor is already defined, should throw a NotSupportedError]
+    expected: FAIL
+