new file mode 100644
--- /dev/null
+++ b/dom/base/AnonymousContent.cpp
@@ -0,0 +1,133 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 "AnonymousContent.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/AnonymousContentBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIDocument.h"
+#include "nsIDOMHTMLCollection.h"
+#include "nsStyledElement.h"
+
+namespace mozilla {
+namespace dom {
+
+// Ref counting and cycle collection
+NS_IMPL_CYCLE_COLLECTION_ROOT_NATIVE(AnonymousContent, AddRef)
+NS_IMPL_CYCLE_COLLECTION_UNROOT_NATIVE(AnonymousContent, Release)
+NS_IMPL_CYCLE_COLLECTION(AnonymousContent, mContentNode)
+
+AnonymousContent::AnonymousContent(Element* aContentNode) :
+  mContentNode(aContentNode)
+{}
+
+AnonymousContent::~AnonymousContent()
+{
+}
+
+nsCOMPtr<Element>
+AnonymousContent::GetContentNode()
+{
+  return mContentNode;
+}
+
+void
+AnonymousContent::SetTextContentForElement(const nsAString& aElementId,
+                                           const nsAString& aText,
+                                           ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  element->SetTextContent(aText, aRv);
+}
+
+void
+AnonymousContent::GetTextContentForElement(const nsAString& aElementId,
+                                           DOMString& aText,
+                                           ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  element->GetTextContent(aText, aRv);
+}
+
+void
+AnonymousContent::SetAttributeForElement(const nsAString& aElementId,
+                                         const nsAString& aName,
+                                         const nsAString& aValue,
+                                         ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  element->SetAttribute(aName, aValue, aRv);
+}
+
+void
+AnonymousContent::GetAttributeForElement(const nsAString& aElementId,
+                                         const nsAString& aName,
+                                         DOMString& aValue,
+                                         ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  element->GetAttribute(aName, aValue);
+}
+
+void
+AnonymousContent::RemoveAttributeForElement(const nsAString& aElementId,
+                                            const nsAString& aName,
+                                            ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  element->RemoveAttribute(aName, aRv);
+}
+
+Element*
+AnonymousContent::GetElementById(const nsAString& aElementId)
+{
+  // This can be made faster in the future if needed.
+  nsCOMPtr<nsIAtom> elementId = do_GetAtom(aElementId);
+  for (nsIContent* kid = mContentNode->GetFirstChild(); kid;
+       kid = kid->GetNextNode(mContentNode)) {
+    if (!kid->IsElement()) {
+      continue;
+    }
+    nsIAtom* id = kid->AsElement()->GetID();
+    if (id && id == elementId) {
+      return kid->AsElement();
+    }
+  }
+  return nullptr;
+}
+
+JSObject*
+AnonymousContent::WrapObject(JSContext* aCx)
+{
+  return AnonymousContentBinding::Wrap(aCx, this);
+}
+
+} // dom namespace
+} // mozilla namespace

new file mode 100644
--- /dev/null
+++ b/dom/base/AnonymousContent.h
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 3; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * 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_AnonymousContent_h
+#define mozilla_dom_AnonymousContent_h
+
+#include "mozilla/dom/Element.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsICSSDeclaration.h"
+#include "nsIDocument.h"
+
+namespace mozilla {
+namespace dom {
+
+class Element;
+
+class AnonymousContent MOZ_FINAL
+{
+public:
+  // Ref counting and cycle collection
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(AnonymousContent)
+  NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(AnonymousContent)
+
+  explicit AnonymousContent(Element* aContentNode);
+  nsCOMPtr<Element> GetContentNode();
+  JSObject* WrapObject(JSContext* aCx);
+
+  // WebIDL methods
+  void SetTextContentForElement(const nsAString& aElementId,
+                                const nsAString& aText,
+                                ErrorResult& aRv);
+
+  void GetTextContentForElement(const nsAString& aElementId,
+                                DOMString& aText,
+                                ErrorResult& aRv);
+
+  void SetAttributeForElement(const nsAString& aElementId,
+                              const nsAString& aName,
+                              const nsAString& aValue,
+                              ErrorResult& aRv);
+
+  void GetAttributeForElement(const nsAString& aElementId,
+                              const nsAString& aName,
+                              DOMString& aValue,
+                              ErrorResult& aRv);
+
+  void RemoveAttributeForElement(const nsAString& aElementId,
+                                 const nsAString& aName,
+                                 ErrorResult& aRv);
+
+private:
+  ~AnonymousContent();
+  Element* GetElementById(const nsAString& aElementId);
+  nsCOMPtr<Element> mContentNode;
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_AnonymousContent_h

--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -137,16 +137,17 @@ if CONFIG['MOZ_WEBRTC']:
     ]
 
 EXPORTS.mozilla += [
     'CORSMode.h',
     'FeedWriterEnabled.h',
 ]
 
 EXPORTS.mozilla.dom += [
+    'AnonymousContent.h',
     'Attr.h',
     'BarProps.h',
     'BlobSet.h',
     'ChildIterator.h',
     'Comment.h',
     'Console.h',
     'DirectionalityUtils.h',
     'DocumentFragment.h',
@@ -191,16 +192,17 @@ EXPORTS.mozilla.dom += [
     'Text.h',
     'TreeWalker.h',
     'URL.h',
     'URLSearchParams.h',
     'WebSocket.h',
 ]
 
 UNIFIED_SOURCES += [
+    'AnonymousContent.cpp',
     'Attr.cpp',
     'BarProps.cpp',
     'ChildIterator.cpp',
     'Comment.cpp',
     'CompositionStringSynthesizer.cpp',
     'Console.cpp',
     'Crypto.cpp',
     'DirectionalityUtils.cpp',

--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -96,16 +96,20 @@ DOMInterfaces = {
 'MozAbortablePromise': {
     'nativeType': 'mozilla::dom::AbortablePromise',
 },
 
 'AbstractWorker': {
     'concrete': False
 },
 
+'AnonymousContent': {
+    'wrapperCache': False
+},
+
 'ArchiveReader': {
     'nativeType': 'mozilla::dom::archivereader::ArchiveReader',
 },
 
 'ArchiveRequest': {
     'nativeType': 'mozilla::dom::archivereader::ArchiveRequest',
 },
 

new file mode 100644
--- /dev/null
+++ b/dom/webidl/AnonymousContent.webidl
@@ -0,0 +1,56 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/.
+ */
+
+/*
+ * This file declares the AnonymousContent interface which is used to
+ * manipulate content that has been inserted into the document's canvasFrame
+ * anonymous container.
+ * See Document.insertAnonymousContent.
+ *
+ * This API never returns a reference to the actual inserted DOM node on
+ * purpose. This is to make sure the content cannot be randomly changed and the
+ * DOM cannot be traversed from the node, so that Gecko can remain in control of
+ * the inserted content.
+ */
+
+[ChromeOnly]
+interface AnonymousContent {
+  /**
+   * Get the text content of an element inside this custom anonymous content.
+   */
+  [Throws]
+  DOMString getTextContentForElement(DOMString elementId);
+
+  /**
+   * Set the text content of an element inside this custom anonymous content.
+   */
+  [Throws]
+  void setTextContentForElement(DOMString elementId, DOMString text);
+
+  /**
+   * Get the value of an attribute of an element inside this custom anonymous
+   * content.
+   */
+  [Throws]
+  DOMString? getAttributeForElement(DOMString elementId,
+                                    DOMString attributeName);
+
+  /**
+   * Set the value of an attribute of an element inside this custom anonymous
+   * content.
+   */
+  [Throws]
+  void setAttributeForElement(DOMString elementId,
+                              DOMString attributeName,
+                              DOMString value);
+
+  /**
+   * Remove an attribute from an element inside this custom anonymous content.
+   */
+  [Throws]
+  void removeAttributeForElement(DOMString elementId,
+                                 DOMString attributeName);
+};

--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -22,16 +22,17 @@ WEBIDL_FILES = [
     'AlarmsManager.webidl',
     'AnalyserNode.webidl',
     'Animatable.webidl',
     'Animation.webidl',
     'AnimationEffect.webidl',
     'AnimationEvent.webidl',
     'AnimationPlayer.webidl',
     'AnimationTimeline.webidl',
+    'AnonymousContent.webidl',
     'AppInfo.webidl',
     'AppNotificationServiceOptions.webidl',
     'Apps.webidl',
     'APZTestData.webidl',
     'ArchiveReader.webidl',
     'ArchiveRequest.webidl',
     'Attr.webidl',
     'AudioBuffer.webidl',