diff --git a/dom/base/ChromeNodeList.cpp b/dom/base/ChromeNodeList.cpp
index ffa101971..2e9f05823 100644
--- a/dom/base/ChromeNodeList.cpp
+++ b/dom/base/ChromeNodeList.cpp
@@ -6,6 +6,7 @@
 
 #include "mozilla/dom/ChromeNodeList.h"
 #include "mozilla/dom/ChromeNodeListBinding.h"
+#include "nsPIDOMWindow.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
diff --git a/dom/base/ChromeNodeList.h b/dom/base/ChromeNodeList.h
index 9908808ac..8dbd42be3 100644
--- a/dom/base/ChromeNodeList.h
+++ b/dom/base/ChromeNodeList.h
@@ -6,6 +6,7 @@
 
 #include "nsCOMArray.h"
 #include "nsContentList.h"
+#include "nsIDocument.h"
 
 namespace mozilla {
 class ErrorResult;
diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
new file mode 100644
index 000000000..8376ab97e
--- /dev/null
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -0,0 +1,104 @@
+/* -*- 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 "DocumentOrShadowRoot.h"
+#include "mozilla/dom/StyleSheetList.h"
+#include "ShadowRoot.h"
+#include "XULDocument.h"
+
+class nsINode;
+class nsIDocument;
+class ShadowRoot;
+
+namespace mozilla {
+namespace dom {
+
+DocumentOrShadowRoot::DocumentOrShadowRoot(mozilla::dom::ShadowRoot* aShadowRoot)
+  : mAsNode(aShadowRoot)
+  , mKind(Kind::ShadowRoot)
+{
+ MOZ_ASSERT(mAsNode);
+}
+
+DocumentOrShadowRoot::DocumentOrShadowRoot(nsIDocument* aDoc)
+  : mAsNode(aDoc)
+  , mKind(Kind::Document)
+{
+  MOZ_ASSERT(mAsNode);
+}
+
+StyleSheetList&
+DocumentOrShadowRoot::EnsureDOMStyleSheets()
+{
+  if (!mDOMStyleSheets) {
+    mDOMStyleSheets = new StyleSheetList(*this);
+  }
+  return *mDOMStyleSheets;
+}
+
+Element*
+DocumentOrShadowRoot::GetElementById(const nsAString& aElementId)
+{
+  if (MOZ_UNLIKELY(aElementId.IsEmpty())) {
+    nsContentUtils::ReportEmptyGetElementByIdArg(AsNode().OwnerDoc());
+    return nullptr;
+  }
+
+  if (nsIdentifierMapEntry* entry = mIdentifierMap.GetEntry(aElementId)) {
+    if (Element* el = entry->GetIdElement()) {
+      return el;
+    }
+  }
+
+  if (MOZ_UNLIKELY(mKind == Kind::Document &&
+      static_cast<nsIDocument&>(AsNode()).IsXULDocument())) {
+    return static_cast<XULDocument&>(AsNode()).GetRefById(aElementId);
+  }
+
+  return nullptr;
+}
+
+already_AddRefed<nsContentList>
+DocumentOrShadowRoot::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                                   const nsAString& aLocalName)
+{
+  ErrorResult rv;
+  RefPtr<nsContentList> list =
+    GetElementsByTagNameNS(aNamespaceURI, aLocalName, rv);
+  if (rv.Failed()) {
+    return nullptr;
+  }
+  return list.forget();
+}
+
+already_AddRefed<nsContentList>
+DocumentOrShadowRoot::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                                   const nsAString& aLocalName,
+                                   mozilla::ErrorResult& aResult)
+{
+  int32_t nameSpaceId = kNameSpaceID_Wildcard;
+
+  if (!aNamespaceURI.EqualsLiteral("*")) {
+    aResult =
+      nsContentUtils::NameSpaceManager()->RegisterNameSpace(aNamespaceURI,
+                                                            nameSpaceId);
+    if (aResult.Failed()) {
+      return nullptr;
+    }
+  }
+
+  NS_ASSERTION(nameSpaceId != kNameSpaceID_Unknown, "Unexpected namespace ID!");
+  return NS_GetContentList(&AsNode(), nameSpaceId, aLocalName);
+}
+
+already_AddRefed<nsContentList>
+DocumentOrShadowRoot::GetElementsByClassName(const nsAString& aClasses)
+{
+  return nsContentUtils::GetElementsByClassName(&AsNode(), aClasses);
+}
+
+}
+}
diff --git a/dom/base/DocumentOrShadowRoot.h b/dom/base/DocumentOrShadowRoot.h
new file mode 100644
index 000000000..59be0c2a9
--- /dev/null
+++ b/dom/base/DocumentOrShadowRoot.h
@@ -0,0 +1,151 @@
+/* -*- 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/. */
+
+#ifndef mozilla_dom_DocumentOrShadowRoot_h__
+#define mozilla_dom_DocumentOrShadowRoot_h__
+
+#include "nsTArray.h"
+#include "nsIdentifierMapEntry.h"
+#include "nsContentListDeclarations.h"
+#include "nsNameSpaceManager.h"
+#include "mozilla/dom/NameSpaceConstants.h"
+
+class nsContentList;
+class nsINode;
+
+namespace mozilla {
+class StyleSheet;
+
+namespace dom {
+
+class StyleSheetList;
+class ShadowRoot;
+
+/**
+ * A class meant to be shared by ShadowRoot and Document, that holds a list of
+ * stylesheets.
+ *
+ * TODO(emilio, bug 1418159): In the future this should hold most of the
+ * relevant style state, this should allow us to fix bug 548397.
+ */
+class DocumentOrShadowRoot
+{
+  enum class Kind {
+    Document,
+    ShadowRoot,
+  };
+
+public:
+  explicit DocumentOrShadowRoot(nsIDocument*);
+  explicit DocumentOrShadowRoot(mozilla::dom::ShadowRoot*);
+
+  nsINode& AsNode()
+  {
+    return *mAsNode;
+  }
+
+  const nsINode& AsNode() const
+  {
+    return *mAsNode;
+  }
+
+  StyleSheet* SheetAt(size_t aIndex) const
+  {
+    return mStyleSheets.SafeElementAt(aIndex);
+  }
+
+  size_t SheetCount() const
+  {
+    return mStyleSheets.Length();
+  }
+
+  int32_t IndexOfSheet(const StyleSheet& aSheet) const
+  {
+    return mStyleSheets.IndexOf(&aSheet);
+  }
+
+  void InsertSheetAt(size_t aIndex, StyleSheet& aSheet)
+  {
+    mStyleSheets.InsertElementAt(aIndex, &aSheet);
+  }
+
+  void RemoveSheet(StyleSheet& aSheet)
+  {
+    mStyleSheets.RemoveElement(&aSheet);
+  }
+
+  void AppendStyleSheet(StyleSheet& aSheet)
+  {
+    mStyleSheets.AppendElement(&aSheet);
+  }
+
+  StyleSheetList& EnsureDOMStyleSheets();
+
+  Element* GetElementById(const nsAString& aElementId);
+
+  /**
+   * This method returns _all_ the elements in this scope which have id
+   * aElementId, if there are any.  Otherwise it returns null.
+   *
+   * This is useful for stuff like QuerySelector optimization and such.
+   */
+  inline const nsTArray<Element*>*
+  GetAllElementsForId(const nsAString& aElementId) const;
+
+  already_AddRefed<nsContentList>
+  GetElementsByTagName(const nsAString& aTagName)
+  {
+    return NS_GetContentList(&AsNode(), kNameSpaceID_Unknown, aTagName);
+  }
+
+  already_AddRefed<nsContentList>
+  GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                         const nsAString& aLocalName);
+
+  already_AddRefed<nsContentList>
+  GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                         const nsAString& aLocalName,
+                         mozilla::ErrorResult&);
+
+  already_AddRefed<nsContentList>
+  GetElementsByClassName(const nsAString& aClasses);
+
+  ~DocumentOrShadowRoot() = default;
+
+protected:
+  nsTArray<RefPtr<mozilla::StyleSheet>> mStyleSheets;
+  RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
+
+  /*
+   * mIdentifierMap works as follows for IDs:
+   * 1) Attribute changes affect the table immediately (removing and adding
+   *    entries as needed).
+   * 2) Removals from the DOM affect the table immediately
+   * 3) Additions to the DOM always update existing entries for names, and add
+   *    new ones for IDs.
+   */
+  nsTHashtable<nsIdentifierMapEntry> mIdentifierMap;
+
+  nsINode* mAsNode;
+  const Kind mKind;
+};
+
+inline const nsTArray<Element*>*
+DocumentOrShadowRoot::GetAllElementsForId(const nsAString& aElementId) const
+{
+  if (aElementId.IsEmpty()) {
+    return nullptr;
+  }
+
+  nsIdentifierMapEntry* entry = mIdentifierMap.GetEntry(aElementId);
+  return entry ? &entry->GetIdElements() : nullptr;
+}
+
+}
+
+}
+
+#endif
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 3d9b80a98..fd4fa7108 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -139,6 +139,7 @@ class EventStateManager;
 
 namespace dom {
 
+struct CustomElementDefinition;
 class Animation;
 class CustomElementRegistry;
 class Link;
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index d6c103e8e..4aab9e435 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -58,6 +58,7 @@ ShadowRoot::ShadowRoot(Element* aElement, bool aClosed,
                        already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
                        nsXBLPrototypeBinding* aProtoBinding)
   : DocumentFragment(aNodeInfo)
+  , DocumentOrShadowRoot(this)
   , mProtoBinding(aProtoBinding)
   , mInsertionPointChanged(false)
   , mIsComposedDocParticipant(false)
@@ -240,7 +241,7 @@ ShadowRoot::InsertSheet(StyleSheet* aSheet,
 
   linkingElement->SetStyleSheet(aSheet); // This sets the ownerNode on the sheet
 
-  MOZ_DIAGNOSTIC_ASSERT(mProtoBinding->SheetCount() == StyleScope::SheetCount());
+  MOZ_DIAGNOSTIC_ASSERT(mProtoBinding->SheetCount() == DocumentOrShadowRoot::SheetCount());
 #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
   // FIXME(emilio, bug 1425759): For now we keep them duplicated, the proto
   // binding will disappear soon (tm).
@@ -278,44 +279,13 @@ void
 ShadowRoot::RemoveSheet(StyleSheet* aSheet)
 {
   mProtoBinding->RemoveStyleSheet(aSheet);
-  StyleScope::RemoveSheet(*aSheet);
+  DocumentOrShadowRoot::RemoveSheet(*aSheet);
 
   if (aSheet->IsApplicable()) {
     StyleSheetChanged();
   }
 }
 
-Element*
-ShadowRoot::GetElementById(const nsAString& aElementId)
-{
-  nsIdentifierMapEntry *entry = mIdentifierMap.GetEntry(aElementId);
-  return entry ? entry->GetIdElement() : nullptr;
-}
-
-already_AddRefed<nsContentList>
-ShadowRoot::GetElementsByTagName(const nsAString& aTagName)
-{
-  return NS_GetContentList(this, kNameSpaceID_Unknown, aTagName);
-}
-
-already_AddRefed<nsContentList>
-ShadowRoot::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
-                                   const nsAString& aLocalName)
-{
-  int32_t nameSpaceId = kNameSpaceID_Wildcard;
-
-  if (!aNamespaceURI.EqualsLiteral("*")) {
-    nsresult rv =
-      nsContentUtils::NameSpaceManager()->RegisterNameSpace(aNamespaceURI,
-                                                            nameSpaceId);
-    NS_ENSURE_SUCCESS(rv, nullptr);
-  }
-
-  NS_ASSERTION(nameSpaceId != kNameSpaceID_Unknown, "Unexpected namespace ID!");
-
-  return NS_GetContentList(this, nameSpaceId, aLocalName);
-}
-
 void
 ShadowRoot::AddToIdTable(Element* aElement, nsIAtom* aId)
 {
@@ -337,12 +307,6 @@ ShadowRoot::RemoveFromIdTable(Element* aElement, nsIAtom* aId)
   }
 }
 
-already_AddRefed<nsContentList>
-ShadowRoot::GetElementsByClassName(const nsAString& aClasses)
-{
-  return nsContentUtils::GetElementsByClassName(this, aClasses);
-}
-
 nsresult
 ShadowRoot::GetEventTargetParent(EventChainPreVisitor& aVisitor)
 {
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index 21c6e1733..eb772d49a 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -8,7 +8,7 @@
 #define mozilla_dom_shadowroot_h__
 
 #include "mozilla/dom/DocumentFragment.h"
-#include "mozilla/dom/StyleScope.h"
+#include "mozilla/dom/DocumentOrShadowRoot.h"
 #include "nsCOMPtr.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsIContentInlines.h"
@@ -29,7 +29,7 @@ namespace dom {
 class Element;
 
 class ShadowRoot final : public DocumentFragment,
-                         public StyleScope,
+                         public DocumentOrShadowRoot,
                          public nsStubMutationObserver
 {
 public:
@@ -57,22 +57,14 @@ public:
     return mMode == ShadowRootMode::Closed;
   }
 
-  // StyleScope.
-  nsINode& AsNode() final
-  {
-    return *this;
-  }
-
   // [deprecated] Shadow DOM v0
-  void AddToIdTable(Element* aElement, nsIAtom* aId);
-  void RemoveFromIdTable(Element* aElement, nsIAtom* aId);
   void InsertSheet(StyleSheet* aSheet, nsIContent* aLinkingContent);
   void RemoveSheet(StyleSheet* aSheet);
   bool ApplyAuthorStyles();
   void SetApplyAuthorStyles(bool aApplyAuthorStyles);
   StyleSheetList* StyleSheets()
   {
-    return &StyleScope::EnsureDOMStyleSheets();
+    return &DocumentOrShadowRoot::EnsureDOMStyleSheets();
   }
 
   /**
@@ -123,15 +115,11 @@ public:
 
   static ShadowRoot* FromNode(nsINode* aNode);
 
+  void AddToIdTable(Element* aElement, nsIAtom* aId);
+  void RemoveFromIdTable(Element* aElement, nsIAtom* aId);
+
   // WebIDL methods.
-  Element* GetElementById(const nsAString& aElementId);
-  already_AddRefed<nsContentList>
-    GetElementsByTagName(const nsAString& aNamespaceURI);
-  already_AddRefed<nsContentList>
-    GetElementsByTagNameNS(const nsAString& aNamespaceURI,
-                           const nsAString& aLocalName);
-  already_AddRefed<nsContentList>
-    GetElementsByClassName(const nsAString& aClasses);
+  using mozilla::dom::DocumentOrShadowRoot::GetElementById;
   void GetInnerHTML(nsAString& aInnerHTML);
   void SetInnerHTML(const nsAString& aInnerHTML, ErrorResult& aError);
   void StyleSheetChanged();
@@ -154,7 +142,6 @@ protected:
   // are in the shadow tree and should be kept alive by its parent.
   nsClassHashtable<nsStringHashKey, nsTArray<mozilla::dom::HTMLSlotElement*>> mSlotMap;
 
-  nsTHashtable<nsIdentifierMapEntry> mIdentifierMap;
   nsXBLPrototypeBinding* mProtoBinding;
 
   // It is necessary to hold a reference to the associated nsXBLBinding
diff --git a/dom/base/StyleScope.cpp b/dom/base/StyleScope.cpp
deleted file mode 100644
index 6c708a8ba..000000000
--- a/dom/base/StyleScope.cpp
+++ /dev/null
@@ -1,27 +0,0 @@
-/* -*- 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 "StyleScope.h"
-#include "mozilla/dom/StyleSheetList.h"
-
-namespace mozilla {
-namespace dom {
-
-StyleScope::~StyleScope()
-{
-}
-
-StyleSheetList&
-StyleScope::EnsureDOMStyleSheets()
-{
-  if (!mDOMStyleSheets) {
-    mDOMStyleSheets = new StyleSheetList(*this);
-  }
-  return *mDOMStyleSheets;
-}
-
-}
-}
diff --git a/dom/base/StyleScope.h b/dom/base/StyleScope.h
deleted file mode 100644
index c8957b069..000000000
--- a/dom/base/StyleScope.h
+++ /dev/null
@@ -1,81 +0,0 @@
-/* -*- 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/. */
-
-#ifndef mozilla_dom_StyleScope_h__
-#define mozilla_dom_StyleScope_h__
-
-#include "nsTArray.h"
-
-class nsINode;
-
-namespace mozilla {
-class StyleSheet;
-
-namespace dom {
-
-class StyleSheetList;
-
-/**
- * A class meant to be shared by ShadowRoot and Document, that holds a list of
- * stylesheets.
- *
- * TODO(emilio, bug 1418159): In the future this should hold most of the
- * relevant style state, this should allow us to fix bug 548397.
- */
-class StyleScope
-{
-public:
-  virtual nsINode& AsNode() = 0;
-
-  const nsINode& AsNode() const
-  {
-    return const_cast<StyleScope&>(*this).AsNode();
-  }
-
-  StyleSheet* SheetAt(size_t aIndex) const
-  {
-    return mStyleSheets.SafeElementAt(aIndex);
-  }
-
-  size_t SheetCount() const
-  {
-    return mStyleSheets.Length();
-  }
-
-  int32_t IndexOfSheet(const StyleSheet& aSheet) const
-  {
-    return mStyleSheets.IndexOf(&aSheet);
-  }
-
-  void InsertSheetAt(size_t aIndex, StyleSheet& aSheet)
-  {
-    mStyleSheets.InsertElementAt(aIndex, &aSheet);
-  }
-
-  void RemoveSheet(StyleSheet& aSheet)
-  {
-    mStyleSheets.RemoveElement(&aSheet);
-  }
-
-  void AppendStyleSheet(StyleSheet& aSheet)
-  {
-    mStyleSheets.AppendElement(&aSheet);
-  }
-
-  StyleSheetList& EnsureDOMStyleSheets();
-
-  ~StyleScope();
-
-protected:
-  nsTArray<RefPtr<mozilla::StyleSheet>> mStyleSheets;
-  RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
-};
-
-}
-
-}
-
-#endif
diff --git a/dom/base/StyleSheetList.cpp b/dom/base/StyleSheetList.cpp
index 42db3179d..2b41473ff 100644
--- a/dom/base/StyleSheetList.cpp
+++ b/dom/base/StyleSheetList.cpp
@@ -48,19 +48,19 @@ StyleSheetList::SlowItem(uint32_t aIndex, nsIDOMStyleSheet** aItem)
 void
 StyleSheetList::NodeWillBeDestroyed(const nsINode* aNode)
 {
-  mStyleScope = nullptr;
+  mDocumentOrShadowRoot = nullptr;
 }
 
-StyleSheetList::StyleSheetList(StyleScope& aScope)
-  : mStyleScope(&aScope)
+StyleSheetList::StyleSheetList(DocumentOrShadowRoot& aScope)
+  : mDocumentOrShadowRoot(&aScope)
 {
-  mStyleScope->AsNode().AddMutationObserver(this);
+  mDocumentOrShadowRoot->AsNode().AddMutationObserver(this);
 }
 
 StyleSheetList::~StyleSheetList()
 {
-  if (mStyleScope) {
-    mStyleScope->AsNode().RemoveMutationObserver(this);
+  if (mDocumentOrShadowRoot) {
+    mDocumentOrShadowRoot->AsNode().RemoveMutationObserver(this);
   }
 }
 
diff --git a/dom/base/StyleSheetList.h b/dom/base/StyleSheetList.h
index ea5c33a98..f8f371a81 100644
--- a/dom/base/StyleSheetList.h
+++ b/dom/base/StyleSheetList.h
@@ -7,7 +7,7 @@
 #ifndef mozilla_dom_StyleSheetList_h
 #define mozilla_dom_StyleSheetList_h
 
-#include "mozilla/dom/StyleScope.h"
+#include "mozilla/dom/DocumentOrShadowRoot.h"
 #include "nsIDOMStyleSheetList.h"
 #include "nsWrapperCache.h"
 #include "nsStubDocumentObserver.h"
@@ -31,28 +31,28 @@ public:
 
   NS_DECL_NSIMUTATIONOBSERVER_NODEWILLBEDESTROYED
 
-  explicit StyleSheetList(StyleScope& aScope);
+  explicit StyleSheetList(DocumentOrShadowRoot& aScope);
 
   virtual JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override final;
 
   nsINode* GetParentObject() const
   {
-    return mStyleScope ? &mStyleScope->AsNode() : nullptr;
+    return mDocumentOrShadowRoot ? &mDocumentOrShadowRoot->AsNode() : nullptr;
   }
 
   uint32_t Length() const
   {
-    return mStyleScope ? mStyleScope->SheetCount() : 0;
+    return mDocumentOrShadowRoot ? mDocumentOrShadowRoot->SheetCount() : 0;
   }
 
   StyleSheet* IndexedGetter(uint32_t aIndex, bool& aFound) const
   {
-    if (!mStyleScope) {
+    if (!mDocumentOrShadowRoot) {
       aFound = false;
       return nullptr;
     }
 
-    StyleSheet* sheet = mStyleScope->SheetAt(aIndex);
+    StyleSheet* sheet = mDocumentOrShadowRoot->SheetAt(aIndex);
     aFound = !!sheet;
     return sheet;
   }
@@ -66,7 +66,7 @@ public:
 protected:
   virtual ~StyleSheetList();
 
-  StyleScope* mStyleScope; // Weak, cleared on "NodeWillBeDestroyed".
+  DocumentOrShadowRoot* mDocumentOrShadowRoot; // Weak, cleared on "NodeWillBeDestroyed".
 };
 
 } // namespace dom
diff --git a/dom/base/moz.build b/dom/base/moz.build
index 5a2d80b97..12d2ee47d 100644
--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -161,6 +161,7 @@ EXPORTS.mozilla.dom += [
     'DirectionalityUtils.h',
     'DocGroup.h',
     'DocumentFragment.h',
+    'DocumentOrShadowRoot.h',
     'DocumentType.h',
     'DOMCursor.h',
     'DOMError.h',
@@ -210,7 +211,6 @@ EXPORTS.mozilla.dom += [
     'SimpleTreeIterator.h',
     'StructuredCloneHolder.h',
     'StructuredCloneTags.h',
-    'StyleScope.h',
     'StyleSheetList.h',
     'SubtleCrypto.h',
     'TabGroup.h',
@@ -239,6 +239,7 @@ SOURCES += [
     'DirectionalityUtils.cpp',
     'DocGroup.cpp',
     'DocumentFragment.cpp',
+    'DocumentOrShadowRoot.cpp',
     'DocumentType.cpp',
     'DOMCursor.cpp',
     'DOMError.cpp',
@@ -353,7 +354,6 @@ SOURCES += [
     'ScreenOrientation.cpp',
     'ShadowRoot.cpp',
     'StructuredCloneHolder.cpp',
-    'StyleScope.cpp',
     'StyleSheetList.cpp',
     'SubtleCrypto.cpp',
     'TabGroup.cpp',
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index c0d81a41e..61d10e022 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -3658,6 +3658,14 @@ nsContentUtils::ReportToConsole(uint32_t aErrorFlags,
                                      aLineNumber, aColumnNumber);
 }
 
+/* static */ void
+nsContentUtils::ReportEmptyGetElementByIdArg(const nsIDocument* aDoc)
+{
+  ReportToConsole(nsIScriptError::warningFlag,
+                  NS_LITERAL_CSTRING("DOM"), aDoc,
+                  nsContentUtils::eDOM_PROPERTIES,
+                  "EmptyGetElementByIdParam");
+}
 
 /* static */ nsresult
 nsContentUtils::ReportToConsoleNonLocalized(const nsAString& aErrorText,
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index b58b0e0e3..8cf105bb3 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -941,6 +941,8 @@ public:
                                   uint32_t aLineNumber = 0,
                                   uint32_t aColumnNumber = 0);
 
+  static void ReportEmptyGetElementByIdArg(const nsIDocument* aDoc);
+
   static void LogMessageToConsole(const char* aMsg);
 
   /**
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index b88b1dfdb..f8b049667 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -300,9 +300,24 @@ GetHttpChannelHelper(nsIChannel* aChannel, nsIHttpChannel** aHttpChannel)
 
 #define NAME_NOT_VALID ((nsSimpleContentList*)1)
 
+nsIdentifierMapEntry::nsIdentifierMapEntry(const nsIdentifierMapEntry::AtomOrString& aKey)
+  : mKey(aKey)
+{}
+
+nsIdentifierMapEntry::nsIdentifierMapEntry(const nsIdentifierMapEntry::AtomOrString* aKey)
+  : mKey(aKey ? *aKey : nullptr)
+{}
+
 nsIdentifierMapEntry::~nsIdentifierMapEntry()
-{
-}
+{}
+
+nsIdentifierMapEntry::nsIdentifierMapEntry(nsIdentifierMapEntry&& aOther)
+  : mKey(mozilla::Move(aOther.mKey))
+  , mIdContentList(mozilla::Move(aOther.mIdContentList))
+  , mNameContentList(mozilla::Move(aOther.mNameContentList))
+  , mChangeCallbacks(mozilla::Move(aOther.mChangeCallbacks))
+  , mImageElement(mozilla::Move(aOther.mImageElement))
+{}
 
 void
 nsIdentifierMapEntry::Traverse(nsCycleCollectionTraversalCallback* aCallback)
@@ -326,6 +341,12 @@ nsIdentifierMapEntry::IsEmpty()
          !mChangeCallbacks && !mImageElement;
 }
 
+bool
+nsIdentifierMapEntry::HasNameElement() const
+{
+  return mNameContentList && mNameContentList->Length() != 0;
+}
+
 Element*
 nsIdentifierMapEntry::GetIdElement()
 {
@@ -1226,6 +1247,7 @@ static already_AddRefed<mozilla::dom::NodeInfo> nullNodeInfo;
 // ==================================================================
 nsIDocument::nsIDocument()
   : nsINode(nullNodeInfo),
+    DocumentOrShadowRoot(this),
     mReferrerPolicySet(false),
     mReferrerPolicy(mozilla::net::RP_Default),
     mBlockAllMixedContent(false),
@@ -3234,12 +3256,6 @@ nsDocument::GetElementsByClassName(const nsAString& aClasses,
   return NS_OK;
 }
 
-already_AddRefed<nsContentList>
-nsIDocument::GetElementsByClassName(const nsAString& aClasses)
-{
-  return nsContentUtils::GetElementsByClassName(this, aClasses);
-}
-
 NS_IMETHODIMP
 nsDocument::ReleaseCapture()
 {
@@ -4729,32 +4745,7 @@ nsDocument::BeginLoad()
 void
 nsDocument::ReportEmptyGetElementByIdArg()
 {
-  nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
-                                  NS_LITERAL_CSTRING("DOM"), this,
-                                  nsContentUtils::eDOM_PROPERTIES,
-                                  "EmptyGetElementByIdParam");
-}
-
-Element*
-nsDocument::GetElementById(const nsAString& aElementId)
-{
-  if (!CheckGetElementByIdArg(aElementId)) {
-    return nullptr;
-  }
-
-  nsIdentifierMapEntry *entry = mIdentifierMap.GetEntry(aElementId);
-  return entry ? entry->GetIdElement() : nullptr;
-}
-
-const nsTArray<Element*>*
-nsDocument::GetAllElementsForId(const nsAString& aElementId) const
-{
-  if (aElementId.IsEmpty()) {
-    return nullptr;
-  }
-
-  nsIdentifierMapEntry *entry = mIdentifierMap.GetEntry(aElementId);
-  return entry ? &entry->GetIdElements() : nullptr;
+  nsContentUtils::ReportEmptyGetElementByIdArg(this);
 }
 
 NS_IMETHODIMP
@@ -5646,27 +5637,6 @@ nsDocument::BlockedTrackingNodes() const
   return list.forget();
 }
 
-already_AddRefed<nsContentList>
-nsIDocument::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
-                                    const nsAString& aLocalName,
-                                    ErrorResult& aResult)
-{
-  int32_t nameSpaceId = kNameSpaceID_Wildcard;
-
-  if (!aNamespaceURI.EqualsLiteral("*")) {
-    aResult =
-      nsContentUtils::NameSpaceManager()->RegisterNameSpace(aNamespaceURI,
-                                                            nameSpaceId);
-    if (aResult.Failed()) {
-      return nullptr;
-    }
-  }
-
-  NS_ASSERTION(nameSpaceId != kNameSpaceID_Unknown, "Unexpected namespace ID!");
-
-  return NS_GetContentList(this, nameSpaceId, aLocalName);
-}
-
 NS_IMETHODIMP
 nsDocument::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
                                    const nsAString& aLocalName,
@@ -5674,7 +5644,7 @@ nsDocument::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
 {
   ErrorResult rv;
   RefPtr<nsContentList> list =
-    nsIDocument::GetElementsByTagNameNS(aNamespaceURI, aLocalName, rv);
+    GetElementsByTagNameNS(aNamespaceURI, aLocalName, rv);
   if (rv.Failed()) {
     return rv.StealNSResult();
   }
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 2e4b0074c..502ba0f13 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -333,7 +333,6 @@ class nsDocument : public nsIDocument,
 
 public:
   typedef mozilla::dom::Element Element;
-  using nsIDocument::GetElementsByTagName;
   typedef mozilla::net::ReferrerPolicy ReferrerPolicy;
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
@@ -631,6 +630,11 @@ public:
   // nsIDOMDocumentXBL
   NS_DECL_NSIDOMDOCUMENTXBL
 
+  using mozilla::dom::DocumentOrShadowRoot::GetElementById;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByTagName;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByTagNameNS;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByClassName;
+
   // nsIDOMEventTarget
   virtual nsresult GetEventTargetParent(
                      mozilla::EventChainPreVisitor& aVisitor) override;
@@ -819,10 +823,7 @@ public:
   virtual void ResetScrolledToRefAlready() override;
   virtual void SetChangeScrollPosWhenScrollingToRef(bool aValue) override;
 
-  virtual Element *GetElementById(const nsAString& aElementId) override;
-  virtual const nsTArray<Element*>* GetAllElementsForId(const nsAString& aElementId) const override;
-
-  virtual Element *LookupImageElement(const nsAString& aElementId) override;
+  virtual Element* LookupImageElement(const nsAString& aElementId) override;
   virtual void MozSetImageElement(const nsAString& aImageElementId,
                                   Element* aElement) override;
 
@@ -1206,14 +1207,6 @@ public:
   RefPtr<nsDOMStyleSheetSetList> mStyleSheetSetList;
   RefPtr<mozilla::dom::ScriptLoader> mScriptLoader;
   nsDocHeaderData* mHeaderData;
-  /* mIdentifierMap works as follows for IDs:
-   * 1) Attribute changes affect the table immediately (removing and adding
-   *    entries as needed).
-   * 2) Removals from the DOM affect the table immediately
-   * 3) Additions to the DOM always update existing entries for names, and add
-   *    new ones for IDs.
-   */
-  nsTHashtable<nsIdentifierMapEntry> mIdentifierMap;
 
   nsClassHashtable<nsStringHashKey, nsRadioGroupStruct> mRadioGroups;
 
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 4d2063ce7..33aac3a3d 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -34,7 +34,7 @@
 #include "prclist.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/CORSMode.h"
-#include "mozilla/dom/StyleScope.h"
+#include "mozilla/dom/DocumentOrShadowRoot.h"
 #include "mozilla/LinkedList.h"
 #include "mozilla/StyleBackendType.h"
 #include "mozilla/StyleSheet.h"
@@ -198,7 +198,7 @@ class nsContentList;
 // Document interface.  This is implemented by all document objects in
 // Gecko.
 class nsIDocument : public nsINode,
-                    public mozilla::dom::StyleScope
+                    public mozilla::dom::DocumentOrShadowRoot
 {
   typedef mozilla::dom::GlobalObject GlobalObject;
 
@@ -499,7 +499,7 @@ public:
    * to remove it.
    */
   typedef bool (* IDTargetObserver)(Element* aOldElement,
-                                      Element* aNewelement, void* aData);
+                                    Element* aNewelement, void* aData);
 
   /**
    * Add an IDTargetObserver for a specific ID. The IDTargetObserver
@@ -1071,14 +1071,9 @@ public:
    */
   virtual void EnsureOnDemandBuiltInUASheet(mozilla::StyleSheet* aSheet) = 0;
 
-  nsINode& AsNode() final
-  {
-    return *this;
-  }
-
   mozilla::dom::StyleSheetList* StyleSheets()
   {
-    return &StyleScope::EnsureDOMStyleSheets();
+    return &DocumentOrShadowRoot::EnsureDOMStyleSheets();
   }
 
   /**
@@ -2362,19 +2357,10 @@ public:
   virtual void ResetScrolledToRefAlready() = 0;
   virtual void SetChangeScrollPosWhenScrollingToRef(bool aValue) = 0;
 
-  /**
-   * This method is similar to GetElementById() from nsIDOMDocument but it
-   * returns a mozilla::dom::Element instead of a nsIDOMElement.
-   * It prevents converting nsIDOMElement to mozilla::dom::Element which is
-   * already converted from mozilla::dom::Element.
-   */
-  virtual Element* GetElementById(const nsAString& aElementId) = 0;
-
-  /**
-   * This method returns _all_ the elements in this document which
-   * have id aElementId, if there are any.  Otherwise it returns null.
-   */
-  virtual const nsTArray<Element*>* GetAllElementsForId(const nsAString& aElementId) const = 0;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementById;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByTagName;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByTagNameNS;
+  using mozilla::dom::DocumentOrShadowRoot::GetElementsByClassName;
 
   /**
    * Lookup an image element using its associated ID, which is usually provided
@@ -2574,18 +2560,6 @@ public:
 
   nsIDocument* GetTopLevelContentDocument();
 
-  already_AddRefed<nsContentList>
-  GetElementsByTagName(const nsAString& aTagName)
-  {
-    return NS_GetContentList(this, kNameSpaceID_Unknown, aTagName);
-  }
-  already_AddRefed<nsContentList>
-    GetElementsByTagNameNS(const nsAString& aNamespaceURI,
-                           const nsAString& aLocalName,
-                           mozilla::ErrorResult& aResult);
-  already_AddRefed<nsContentList>
-    GetElementsByClassName(const nsAString& aClasses);
-  // GetElementById defined above
   virtual already_AddRefed<Element>
     CreateElement(const nsAString& aTagName,
                   const mozilla::dom::ElementCreationOptionsOrString& aOptions,
diff --git a/dom/base/nsIdentifierMapEntry.h b/dom/base/nsIdentifierMapEntry.h
index 41b8b4a83..119a7e453 100644
--- a/dom/base/nsIdentifierMapEntry.h
+++ b/dom/base/nsIdentifierMapEntry.h
@@ -15,18 +15,24 @@
 
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Move.h"
-#include "mozilla/dom/Element.h"
 #include "mozilla/net/ReferrerPolicy.h"
 
 #include "nsCOMArray.h"
 #include "nsCOMPtr.h"
-#include "nsContentList.h"
 #include "nsIAtom.h"
-#include "nsIDocument.h"
 #include "nsTArray.h"
 #include "nsTHashtable.h"
+#include "nsHashKeys.h"
 
 class nsIContent;
+class nsContentList;
+class nsBaseContentList;
+
+namespace mozilla {
+namespace dom {
+  class Element;
+} // namespace dom
+} // namespace mozilla
 
 /**
  * Right now our identifier map entries contain information for 'name'
@@ -42,6 +48,16 @@ class nsIContent;
  */
 class nsIdentifierMapEntry : public PLDHashEntryHdr
 {
+  typedef mozilla::dom::Element Element;
+  typedef mozilla::net::ReferrerPolicy ReferrerPolicy;
+
+  /**
+   * @see nsIDocument::IDTargetObserver, this is just here to avoid include
+   * hell.
+   */
+  typedef bool (* IDTargetObserver)(Element* aOldElement,
+                                    Element* aNewelement, void* aData);
+
 public:
   struct AtomOrString
   {
@@ -66,25 +82,9 @@ public:
   typedef const AtomOrString& KeyType;
   typedef const AtomOrString* KeyTypePointer;
 
-  typedef mozilla::dom::Element Element;
-  typedef mozilla::net::ReferrerPolicy ReferrerPolicy;
-
-  explicit nsIdentifierMapEntry(const AtomOrString& aKey)
-    : mKey(aKey)
-  {
-  }
-  explicit nsIdentifierMapEntry(const AtomOrString* aKey)
-    : mKey(aKey ? *aKey : nullptr)
-  {
-  }
-  nsIdentifierMapEntry(nsIdentifierMapEntry&& aOther) :
-    mKey(mozilla::Move(aOther.GetKey())),
-    mIdContentList(mozilla::Move(aOther.mIdContentList)),
-    mNameContentList(aOther.mNameContentList.forget()),
-    mChangeCallbacks(aOther.mChangeCallbacks.forget()),
-    mImageElement(aOther.mImageElement.forget())
-  {
-  }
+  explicit nsIdentifierMapEntry(const AtomOrString& aKey);
+  explicit nsIdentifierMapEntry(const AtomOrString* aKey);
+  nsIdentifierMapEntry(nsIdentifierMapEntry&& aOther);
   ~nsIdentifierMapEntry();
 
   KeyType GetKey() const { return mKey; }
@@ -131,9 +131,7 @@ public:
   nsBaseContentList* GetNameContentList() {
     return mNameContentList;
   }
-  bool HasNameElement() const {
-    return mNameContentList && mNameContentList->Length() != 0;
-  }
+  bool HasNameElement() const;
 
   /**
    * Returns the element if we know the element associated with this
@@ -173,9 +171,9 @@ public:
   bool HasIdElementExposedAsHTMLDocumentProperty();
 
   bool HasContentChangeCallback() { return mChangeCallbacks != nullptr; }
-  void AddContentChangeCallback(nsIDocument::IDTargetObserver aCallback,
+  void AddContentChangeCallback(IDTargetObserver aCallback,
                                 void* aData, bool aForImage);
-  void RemoveContentChangeCallback(nsIDocument::IDTargetObserver aCallback,
+  void RemoveContentChangeCallback(IDTargetObserver aCallback,
                                 void* aData, bool aForImage);
 
   /**
@@ -186,7 +184,7 @@ public:
   void Traverse(nsCycleCollectionTraversalCallback* aCallback);
 
   struct ChangeCallback {
-    nsIDocument::IDTargetObserver mCallback;
+    IDTargetObserver mCallback;
     void* mData;
     bool mForImage;
   };
diff --git a/dom/base/nsNameSpaceManager.h b/dom/base/nsNameSpaceManager.h
index d5c3a25fe..e6dc67a3b 100644
--- a/dom/base/nsNameSpaceManager.h
+++ b/dom/base/nsNameSpaceManager.h
@@ -10,13 +10,13 @@
 #include "nsDataHashtable.h"
 #include "nsHashKeys.h"
 #include "nsIAtom.h"
-#include "nsIDocument.h"
 #include "nsIObserver.h"
 #include "nsTArray.h"
 
 #include "mozilla/StaticPtr.h"
 
 class nsAString;
+class nsIDocument;
 
 /**
  * The Name Space Manager tracks the association between a NameSpace
diff --git a/dom/html/nsHTMLDocument.h b/dom/html/nsHTMLDocument.h
index c9e46b3fa..4f44befbb 100644
--- a/dom/html/nsHTMLDocument.h
+++ b/dom/html/nsHTMLDocument.h
@@ -155,10 +155,7 @@ public:
 
   virtual void RemovedFromDocShell() override;
 
-  virtual mozilla::dom::Element *GetElementById(const nsAString& aElementId) override
-  {
-    return nsDocument::GetElementById(aElementId);
-  }
+  using mozilla::dom::DocumentOrShadowRoot::GetElementById;
 
   virtual void DocAddSizeOfExcludingThis(nsWindowSizes* aWindowSizes) const override;
   // DocAddSizeOfIncludingThis is inherited from nsIDocument.
diff --git a/dom/xul/XULDocument.cpp b/dom/xul/XULDocument.cpp
index 9d5d33636..c5ec9a208 100644
--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -1578,24 +1578,13 @@ XULDocument::GetCommandDispatcher(nsIDOMXULCommandDispatcher** aTracker)
 }
 
 Element*
-XULDocument::GetElementById(const nsAString& aId)
+XULDocument::GetRefById(const nsAString& aID)
 {
-    if (!CheckGetElementByIdArg(aId))
-        return nullptr;
-
-    nsIdentifierMapEntry *entry = mIdentifierMap.GetEntry(aId);
-    if (entry) {
-        Element* element = entry->GetIdElement();
-        if (element)
-            return element;
-    }
-
-    nsRefMapEntry* refEntry = mRefMap.GetEntry(aId);
-    if (refEntry) {
-        NS_ASSERTION(refEntry->GetFirstElement(),
-                     "nsRefMapEntries should have nonempty content lists");
+    if (nsRefMapEntry* refEntry = mRefMap.GetEntry(aID)) {
+        MOZ_ASSERT(refEntry->GetFirstElement());
         return refEntry->GetFirstElement();
     }
+
     return nullptr;
 }
 
diff --git a/dom/xul/XULDocument.h b/dom/xul/XULDocument.h
index e72edfeed..5c4dad42e 100644
--- a/dom/xul/XULDocument.h
+++ b/dom/xul/XULDocument.h
@@ -148,6 +148,7 @@ public:
     using nsDocument::CreateElementNS;
     NS_FORWARD_NSIDOMDOCUMENT(XMLDocument::)
     // And explicitly import the things from nsDocument that we just shadowed
+    using mozilla::dom::DocumentOrShadowRoot::GetElementById;
     using nsDocument::GetImplementation;
     using nsDocument::GetTitle;
     using nsDocument::SetTitle;
@@ -156,8 +157,8 @@ public:
     using nsDocument::GetMozFullScreenElement;
     using nsIDocument::GetLocation;
 
-    // nsDocument interface overrides
-    virtual Element* GetElementById(const nsAString & elementId) override;
+    // Helper for StyleScope::GetElementById.
+    Element* GetRefById(const nsAString & elementId);
 
     // nsIDOMXULDocument interface
     NS_DECL_NSIDOMXULDOCUMENT
diff --git a/editor/libeditor/TextEditor.cpp b/editor/libeditor/TextEditor.cpp
index c3cfa4a72..07b06a96a 100644
--- a/editor/libeditor/TextEditor.cpp
+++ b/editor/libeditor/TextEditor.cpp
@@ -24,6 +24,7 @@
 #include "nsCharTraits.h"
 #include "nsComponentManagerUtils.h"
 #include "nsContentCID.h"
+#include "nsContentList.h"
 #include "nsCopySupport.h"
 #include "nsDebug.h"
 #include "nsDependentSubstring.h"