--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -5726,25 +5726,46 @@ nsDocument::IsWebComponentsEnabled(JSCon
   }
 
   // Check for the webcomponents permission. See Bug 1181555.
   JSAutoCompartment ac(aCx, obj);
   JS::Rooted<JSObject*> global(aCx, JS_GetGlobalForObject(aCx, obj));
   nsCOMPtr<nsPIDOMWindowInner> window =
     do_QueryInterface(nsJSUtils::GetStaticScriptGlobal(global));
 
-  if (window) {
+  return IsWebComponentsEnabled(window);
+}
+
+bool
+nsDocument::IsWebComponentsEnabled(dom::NodeInfo* aNodeInfo)
+{
+  if (Preferences::GetBool("dom.webcomponents.enabled")) {
+    return true;
+  }
+
+  nsIDocument* doc = aNodeInfo->GetDocument();
+  // Use GetScopeObject() here so that data documents work the same way as the
+  // main document they're associated with.
+  nsCOMPtr<nsPIDOMWindowInner> window =
+    do_QueryInterface(doc->GetScopeObject());
+  return IsWebComponentsEnabled(window);
+}
+
+bool
+nsDocument::IsWebComponentsEnabled(nsPIDOMWindowInner* aWindow)
+{
+  if (aWindow) {
     nsresult rv;
     nsCOMPtr<nsIPermissionManager> permMgr =
       do_GetService(NS_PERMISSIONMANAGER_CONTRACTID, &rv);
     NS_ENSURE_SUCCESS(rv, false);
 
     uint32_t perm;
     rv = permMgr->TestPermissionFromWindow(
-      window, "moz-extremely-unstable-and-will-change-webcomponents", &perm);
+      aWindow, "moz-extremely-unstable-and-will-change-webcomponents", &perm);
     NS_ENSURE_SUCCESS(rv, false);
 
     return perm == nsIPermissionManager::ALLOW_ACTION;
   }
 
   return false;
 }
 

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -1377,17 +1377,23 @@ private:
     const nsAString& aLocalName,
     uint32_t aNamespaceID,
     ErrorResult& rv);
 
 public:
   virtual already_AddRefed<mozilla::dom::CustomElementRegistry>
     GetCustomElementRegistry() override;
 
+  // Check whether web components are enabled for the global of aObject.
   static bool IsWebComponentsEnabled(JSContext* aCx, JSObject* aObject);
+  // Check whether web components are enabled for the global of the document
+  // this nodeinfo comes from.
+  static bool IsWebComponentsEnabled(mozilla::dom::NodeInfo* aNodeInfo);
+  // Check whether web components are enabled for the given window.
+  static bool IsWebComponentsEnabled(nsPIDOMWindowInner* aWindow);
 
   RefPtr<mozilla::EventListenerManager> mListenerManager;
   RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
   RefPtr<nsDOMStyleSheetSetList> mStyleSheetSetList;
   RefPtr<nsScriptLoader> mScriptLoader;
   nsDocHeaderData* mHeaderData;
   /* mIdentifierMap works as follows for IDs:
    * 1) Attribute changes affect the table immediately (removing and adding

--- a/dom/html/HTMLContentElement.cpp
+++ b/dom/html/HTMLContentElement.cpp
@@ -1,29 +1,50 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* 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/. */
 
 #include "mozilla/dom/HTMLContentElement.h"
 #include "mozilla/dom/HTMLContentElementBinding.h"
+#include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/NodeListBinding.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "mozilla/css/StyleRule.h"
 #include "nsGkAtoms.h"
 #include "nsStyleConsts.h"
 #include "nsIAtom.h"
 #include "nsCSSRuleProcessor.h"
 #include "nsRuleData.h"
 #include "nsRuleProcessorData.h"
 #include "nsRuleWalker.h"
 #include "nsCSSParser.h"
+#include "nsDocument.h"
 
-NS_IMPL_NS_NEW_HTML_ELEMENT(Content)
+// Expand NS_IMPL_NS_NEW_HTML_ELEMENT(Content) to add check for web components
+// being enabled.
+nsGenericHTMLElement*
+NS_NewHTMLContentElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
+                         mozilla::dom::FromParser aFromParser)
+{
+  // When this check is removed, remove the nsDocument.h and
+  // HTMLUnknownElement.h includes.  Also remove nsINode::IsHTMLContentElement.
+  //
+  // We have to jump through some hoops to be able to produce both NodeInfo* and
+  // already_AddRefed<NodeInfo>& for our callees.
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
+  if (!nsDocument::IsWebComponentsEnabled(nodeInfo)) {
+    already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
+    return new mozilla::dom::HTMLUnknownElement(nodeInfoArg);
+  }
+
+  already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
+  return new mozilla::dom::HTMLContentElement(nodeInfoArg);
+}
 
 using namespace mozilla::dom;
 
 HTMLContentElement::HTMLContentElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo)
   : nsGenericHTMLElement(aNodeInfo), mValidSelector(true), mIsInsertionPoint(false)
 {
 }
 

--- a/dom/html/HTMLShadowElement.cpp
+++ b/dom/html/HTMLShadowElement.cpp
@@ -3,20 +3,41 @@
 /* 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/. */
 
 #include "mozilla/dom/ShadowRoot.h"
 
 #include "ChildIterator.h"
 #include "nsContentUtils.h"
+#include "nsDocument.h"
 #include "mozilla/dom/HTMLShadowElement.h"
+#include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/HTMLShadowElementBinding.h"
 
-NS_IMPL_NS_NEW_HTML_ELEMENT(Shadow)
+// Expand NS_IMPL_NS_NEW_HTML_ELEMENT(Shadow) to add check for web components
+// being enabled.
+nsGenericHTMLElement*
+NS_NewHTMLShadowElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
+                        mozilla::dom::FromParser aFromParser)
+{
+  // When this check is removed, remove the nsDocument.h and
+  // HTMLUnknownElement.h includes.  Also remove nsINode::IsHTMLShadowElement.
+  //
+  // We have to jump through some hoops to be able to produce both NodeInfo* and
+  // already_AddRefed<NodeInfo>& for our callees.
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
+  if (!nsDocument::IsWebComponentsEnabled(nodeInfo)) {
+    already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
+    return new mozilla::dom::HTMLUnknownElement(nodeInfoArg);
+  }
+
+  already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
+  return new mozilla::dom::HTMLShadowElement(nodeInfoArg);
+}
 
 using namespace mozilla::dom;
 
 HTMLShadowElement::HTMLShadowElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo)
   : nsGenericHTMLElement(aNodeInfo), mIsInsertionPoint(false)
 {
 }