diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 098db6e57084..3bca30252ea9 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -38,8 +38,6 @@ CustomElementCallback::Call()
       nsIDocument* document = mThisObject->GetComposedDoc();
       if (document) {
         NodeInfo* ni = mThisObject->NodeInfo();
-        nsDependentAtomString extType(mOwnerData->mType);
-
         // We need to do this because at this point, CustomElementDefinition is
         // not set to CustomElementData yet, so EnqueueLifecycleCallback will
         // fail to find the CE definition for this custom element.
@@ -47,7 +45,7 @@ CustomElementCallback::Call()
         CustomElementDefinition* definition =
           nsContentUtils::LookupCustomElementDefinition(document,
             ni->LocalName(), ni->NamespaceID(),
-            extType.IsEmpty() ? nullptr : &extType);
+            mOwnerData->GetCustomElementType());
 
         nsContentUtils::EnqueueLifecycleCallback(
           nsIDocument::eConnected, mThisObject, nullptr, nullptr, definition);
@@ -132,10 +130,10 @@ CustomElementData::CustomElementData(nsAtom* aType)
 }
 
 CustomElementData::CustomElementData(nsAtom* aType, State aState)
-  : mType(aType)
-  , mElementIsBeingCreated(false)
+  : mElementIsBeingCreated(false)
   , mCreatedCallbackInvoked(true)
   , mState(aState)
+  , mType(aType)
 {
 }
 
@@ -158,6 +156,12 @@ CustomElementData::GetCustomElementDefinition()
   return mCustomElementDefinition;
 }
 
+nsAtom*
+CustomElementData::GetCustomElementType()
+{
+  return mType;
+}
+
 void
 CustomElementData::Traverse(nsCycleCollectionTraversalCallback& aCb) const
 {
@@ -269,12 +273,10 @@ CustomElementRegistry::~CustomElementRegistry()
 
 CustomElementDefinition*
 CustomElementRegistry::LookupCustomElementDefinition(const nsAString& aLocalName,
-                                                     const nsAString* aIs) const
+                                                     nsAtom* aTypeAtom) const
 {
   RefPtr<nsAtom> localNameAtom = NS_Atomize(aLocalName);
-  RefPtr<nsAtom> typeAtom = aIs ? NS_Atomize(*aIs) : localNameAtom;
-
-  CustomElementDefinition* data = mCustomDefinitions.GetWeak(typeAtom);
+  CustomElementDefinition* data = mCustomDefinitions.GetWeak(aTypeAtom);
   if (data && data->mLocalName == localNameAtom) {
     return data;
   }
@@ -322,50 +324,6 @@ CustomElementRegistry::RegisterUnresolvedElement(Element* aElement, nsAtom* aTyp
   aElement->AddStates(NS_EVENT_STATE_UNRESOLVED);
 }
 
-void
-CustomElementRegistry::SetupCustomElement(Element* aElement,
-                                          const nsAString* aTypeExtension)
-{
-  RefPtr<nsAtom> tagAtom = aElement->NodeInfo()->NameAtom();
-  RefPtr<nsAtom> typeAtom = aTypeExtension ?
-    NS_Atomize(*aTypeExtension) : tagAtom;
-
-  if (aTypeExtension && !aElement->HasAttr(kNameSpaceID_None, nsGkAtoms::is)) {
-    // Custom element setup in the parser happens after the "is"
-    // attribute is added.
-    aElement->SetAttr(kNameSpaceID_None, nsGkAtoms::is, *aTypeExtension, true);
-  }
-
-  // SetupCustomElement() should be called with an element that don't have
-  // CustomElementData setup, if not we will hit the assertion in
-  // SetCustomElementData().
-  aElement->SetCustomElementData(new CustomElementData(typeAtom));
-
-  CustomElementDefinition* definition = LookupCustomElementDefinition(
-    aElement->NodeInfo()->LocalName(), aTypeExtension);
-
-  if (!definition) {
-    // The type extension doesn't exist in the registry,
-    // thus we don't need to enqueue callback or adjust
-    // the "is" attribute, but it is possibly an upgrade candidate.
-    RegisterUnresolvedElement(aElement, typeAtom);
-    return;
-  }
-
-  if (definition->mLocalName != tagAtom) {
-    // The element doesn't match the local name for the
-    // definition, thus the element isn't a custom element
-    // and we don't need to do anything more.
-    return;
-  }
-
-  // Enqueuing the created callback will set the CustomElementData on the
-  // element, causing prototype swizzling to occur in Element::WrapObject.
-  // We make it synchronously for createElement/createElementNS in order to
-  // pass tests. It'll be removed when we deprecate custom elements v0.
-  SyncInvokeReactions(nsIDocument::eCreated, aElement, definition);
-}
-
 /* static */ UniquePtr<CustomElementCallback>
 CustomElementRegistry::CreateCustomElementCallback(
   nsIDocument::ElementCallbackType aType, Element* aCustomElement,
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 6e2d2b2f7702..a15001920d68 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -114,9 +114,7 @@ struct CustomElementData
 
   explicit CustomElementData(nsAtom* aType);
   CustomElementData(nsAtom* aType, State aState);
-  // Custom element type, for <button is="x-button"> or <x-button>
-  // this would be x-button.
-  RefPtr<nsAtom> mType;
+
   // Element is being created flag as described in the custom elements spec.
   bool mElementIsBeingCreated;
   // Flag to determine if the created callback has been invoked, thus it
@@ -134,6 +132,7 @@ struct CustomElementData
 
   void SetCustomElementDefinition(CustomElementDefinition* aDefinition);
   CustomElementDefinition* GetCustomElementDefinition();
+  nsAtom* GetCustomElementType();
 
   void Traverse(nsCycleCollectionTraversalCallback& aCb) const;
   void Unlink();
@@ -141,6 +140,9 @@ struct CustomElementData
 private:
   virtual ~CustomElementData() {}
 
+  // Custom element type, for <button is="x-button"> or <x-button>
+  // this would be x-button.
+  RefPtr<nsAtom> mType;
   RefPtr<CustomElementDefinition> mCustomElementDefinition;
 };
 
@@ -370,18 +372,11 @@ public:
    * https://html.spec.whatwg.org/#look-up-a-custom-element-definition
    */
   CustomElementDefinition* LookupCustomElementDefinition(
-    const nsAString& aLocalName, const nsAString* aIs = nullptr) const;
+    const nsAString& aLocalName, nsAtom* aTypeAtom) const;
 
   CustomElementDefinition* LookupCustomElementDefinition(
     JSContext* aCx, JSObject *aConstructor) const;
 
-  /**
-   * Enqueue created callback or register upgrade candidate for
-   * newly created custom elements, possibly extending an existing type.
-   * ex. <x-button>, <button is="x-button> (type extension)
-   */
-  void SetupCustomElement(Element* aElement, const nsAString* aTypeExtension);
-
   static void EnqueueLifecycleCallback(nsIDocument::ElementCallbackType aType,
                                        Element* aCustomElement,
                                        LifecycleCallbackArgs* aArgs,
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 84e2fbbdda13..5efa19c44917 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -524,7 +524,7 @@ Element::WrapObject(JSContext *aCx, JS::Handle<JSObject*> aGivenProto)
     if (data) {
       // If this is a registered custom element then fix the prototype.
       nsContentUtils::GetCustomPrototype(OwnerDoc(), NodeInfo()->NamespaceID(),
-                                         data->mType, &customProto);
+                                         data->GetCustomElementType(), &customProto);
       if (customProto &&
           NodePrincipal()->SubsumesConsideringDomain(nsContentUtils::ObjectPrincipal(customProto))) {
         // Just go ahead and create with the right proto up front.  Set
@@ -2771,7 +2771,7 @@ Element::SetAttrAndNotify(int32_t aNamespaceID,
     if (CustomElementData* data = GetCustomElementData()) {
       if (CustomElementDefinition* definition =
             nsContentUtils::GetElementDefinitionIfObservingAttr(this,
-                                                                data->mType,
+                                                                data->GetCustomElementType(),
                                                                 aName)) {
         MOZ_ASSERT(data->mState == CustomElementData::State::eCustom,
                    "AttributeChanged callback should fire only if "
@@ -3078,7 +3078,7 @@ Element::UnsetAttr(int32_t aNameSpaceID, nsAtom* aName,
     if (CustomElementData* data = GetCustomElementData()) {
       if (CustomElementDefinition* definition =
             nsContentUtils::GetElementDefinitionIfObservingAttr(this,
-                                                                data->mType,
+                                                                data->GetCustomElementType(),
                                                                 aName)) {
         MOZ_ASSERT(data->mState == CustomElementData::State::eCustom,
                    "AttributeChanged callback should fire only if "
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 81f26ae45489..64b4c5f2dd93 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -10123,7 +10123,7 @@ nsContentUtils::HttpsStateIsModern(nsIDocument* aDocument)
 nsContentUtils::LookupCustomElementDefinition(nsIDocument* aDoc,
                                               const nsAString& aLocalName,
                                               uint32_t aNameSpaceID,
-                                              const nsAString* aIs)
+                                              nsAtom* aTypeAtom)
 {
   MOZ_ASSERT(aDoc);
 
@@ -10142,37 +10142,7 @@ nsContentUtils::LookupCustomElementDefinition(nsIDocument* aDoc,
     return nullptr;
   }
 
-  return registry->LookupCustomElementDefinition(aLocalName, aIs);
-}
-
-/* static */ void
-nsContentUtils::SetupCustomElement(Element* aElement,
-                                   const nsAString* aTypeExtension)
-{
-  MOZ_ASSERT(aElement);
-
-  nsCOMPtr<nsIDocument> doc = aElement->OwnerDoc();
-
-  if (!doc) {
-    return;
-  }
-
-  if (aElement->GetNameSpaceID() != kNameSpaceID_XHTML ||
-      !doc->GetDocShell()) {
-    return;
-  }
-
-  nsCOMPtr<nsPIDOMWindowInner> window(doc->GetInnerWindow());
-  if (!window) {
-    return;
-  }
-
-  RefPtr<CustomElementRegistry> registry(window->CustomElements());
-  if (!registry) {
-    return;
-  }
-
-  return registry->SetupCustomElement(aElement, aTypeExtension);
+  return registry->LookupCustomElementDefinition(aLocalName, aTypeAtom);
 }
 
 /* static */ CustomElementDefinition*
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 0427906610a6..f1a8e1ac7824 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -3037,10 +3037,8 @@ public:
     LookupCustomElementDefinition(nsIDocument* aDoc,
                                   const nsAString& aLocalName,
                                   uint32_t aNameSpaceID,
-                                  const nsAString* aIs = nullptr);
+                                  nsAtom* aTypeAtom);
 
-  static void SetupCustomElement(Element* aElement,
-                                 const nsAString* aTypeExtension = nullptr);
 
   static mozilla::dom::CustomElementDefinition*
   GetElementDefinitionIfObservingAttr(Element* aCustomElement,
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 97ae7d142f46..c6fde9cd7014 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -485,7 +485,7 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
           nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
                                                         nodeInfo->LocalName(),
                                                         nodeInfo->NamespaceID(),
-                                                        extension.IsEmpty() ? nullptr : &extension);
+                                                        typeAtom);
         if (definition) {
           nsContentUtils::EnqueueUpgradeReaction(cloneElem, definition);
         }
diff --git a/dom/html/nsHTMLContentSink.cpp b/dom/html/nsHTMLContentSink.cpp
index 16087d6e7975..9e350ed5d2d0 100644
--- a/dom/html/nsHTMLContentSink.cpp
+++ b/dom/html/nsHTMLContentSink.cpp
@@ -259,6 +259,8 @@ NS_NewHTMLElement(Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&&
   RefPtr<mozilla::dom::NodeInfo> nodeInfo = aNodeInfo;
 
   nsAtom *name = nodeInfo->NameAtom();
+  RefPtr<nsAtom> tagAtom = nodeInfo->NameAtom();
+  RefPtr<nsAtom> typeAtom = aIs ? NS_Atomize(*aIs) : tagAtom;
 
   NS_ASSERTION(nodeInfo->NamespaceEquals(kNameSpaceID_XHTML),
                "Trying to HTML elements that don't have the XHTML namespace");
@@ -275,7 +277,7 @@ NS_NewHTMLElement(Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&&
       nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
                                                     nodeInfo->LocalName(),
                                                     nodeInfo->NamespaceID(),
-                                                    aIs);
+                                                    typeAtom);
   }
 
   // It might be a problem that parser synchronously calls constructor, so filed
@@ -318,8 +320,6 @@ NS_NewHTMLElement(Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&&
       // SetupCustomElement() should be called with an element that don't have
       // CustomElementData setup, if not we will hit the assertion in
       // SetCustomElementData().
-      RefPtr<nsAtom> tagAtom = nodeInfo->NameAtom();
-      RefPtr<nsAtom> typeAtom = aIs ? NS_Atomize(*aIs) : tagAtom;
       // Built-in element
       *aResult = CreateHTMLElement(tag, nodeInfo.forget(), aFromParser).take();
       (*aResult)->SetCustomElementData(new CustomElementData(typeAtom));
@@ -369,7 +369,7 @@ NS_NewHTMLElement(Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&&
 
   if (CustomElementRegistry::IsCustomElementEnabled() &&
       (isCustomElementName || aIs)) {
-    nsContentUtils::SetupCustomElement(*aResult, aIs);
+    (*aResult)->SetCustomElementData(new CustomElementData(typeAtom));
   }
 
   return NS_OK;
diff --git a/parser/html/nsHtml5TreeOperation.cpp b/parser/html/nsHtml5TreeOperation.cpp
index c1010502a15a..a01576d5e38a 100644
--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -407,9 +407,12 @@ nsHtml5TreeOperation::CreateHTMLElement(
 
     isCustomElement = (aCreator == NS_NewCustomElement || !isValue.IsEmpty());
     if (isCustomElement && aFromParser != dom::FROM_PARSER_FRAGMENT) {
+      RefPtr<nsAtom> tagAtom = nodeInfo->NameAtom();
+      RefPtr<nsAtom> typeAtom =
+        isValue.IsEmpty() ? tagAtom : NS_Atomize(isValue);
+
       definition = nsContentUtils::LookupCustomElementDefinition(document,
-        nodeInfo->LocalName(), nodeInfo->NamespaceID(),
-        (isValue.IsEmpty() ? nullptr : &isValue));
+        nodeInfo->LocalName(), nodeInfo->NamespaceID(), typeAtom);
 
       if (definition) {
         willExecuteScript = true;