diff --git a/devtools/client/shared/widgets/VariablesView.jsm b/devtools/client/shared/widgets/VariablesView.jsm
index fb72b34d9113..e3f176f6b8f4 100644
--- a/devtools/client/shared/widgets/VariablesView.jsm
+++ b/devtools/client/shared/widgets/VariablesView.jsm
@@ -986,7 +986,7 @@ VariablesView.prototype = {
 
   /**
    * Gets the owner document holding this view.
-   * @return nsIHTMLDocument
+   * @return HTMLDocument
    */
   get document() {
     return this._document || (this._document = this._parent.ownerDocument);
@@ -2048,7 +2048,7 @@ Scope.prototype = {
 
   /**
    * Gets the owner document holding this scope.
-   * @return nsIHTMLDocument
+   * @return HTMLDocument
    */
   get document() {
     return this._document || (this._document = this.ownerView.document);
diff --git a/docshell/base/nsDocShellEditorData.h b/docshell/base/nsDocShellEditorData.h
index 03a981e3a4e9..5bbb140aa586 100644
--- a/docshell/base/nsDocShellEditorData.h
+++ b/docshell/base/nsDocShellEditorData.h
@@ -45,7 +45,7 @@ class nsDocShellEditorData {
   // If this frame is editable, store HTML editor here. It's owned here.
   RefPtr<mozilla::HTMLEditor> mHTMLEditor;
 
-  // Backup for the corresponding nsIHTMLDocument's  editing state while
+  // Backup for the corresponding HTMLDocument's  editing state while
   // the editor is detached.
   mozilla::dom::Document::EditingState mDetachedEditingState;
 
diff --git a/dom/base/WindowNamedPropertiesHandler.cpp b/dom/base/WindowNamedPropertiesHandler.cpp
index 936d7cab109f..623b36b0683c 100644
--- a/dom/base/WindowNamedPropertiesHandler.cpp
+++ b/dom/base/WindowNamedPropertiesHandler.cpp
@@ -117,11 +117,11 @@ bool WindowNamedPropertiesHandler::getOwnPropDescriptor(
   }
 
   // The rest of this function is for HTML documents only.
-  nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(win->GetExtantDoc());
-  if (!htmlDoc) {
+  Document* doc = win->GetExtantDoc();
+  if (!doc || !doc->IsHTMLOrXHTML()) {
     return true;
   }
-  nsHTMLDocument* document = static_cast<nsHTMLDocument*>(htmlDoc.get());
+  nsHTMLDocument* document = doc->AsHTMLDocument();
 
   JS::Rooted<JS::Value> v(aCx);
   Element* element = document->GetElementById(str);
@@ -197,11 +197,11 @@ bool WindowNamedPropertiesHandler::ownPropNames(
   }
 
   names.Clear();
-  nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(win->GetExtantDoc());
-  if (!htmlDoc) {
+  Document* doc = win->GetExtantDoc();
+  if (!doc || !doc->IsHTMLOrXHTML()) {
     return true;
   }
-  nsHTMLDocument* document = static_cast<nsHTMLDocument*>(htmlDoc.get());
+  nsHTMLDocument* document = doc->AsHTMLDocument();
   // Document names are enumerable, so we want to get them no matter what flags
   // is.
   document->GetSupportedNames(names);
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 6aba9d135898..3de975c98582 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -158,7 +158,6 @@
 #include "nsIForm.h"
 #include "nsIFragmentContentSink.h"
 #include "nsContainerFrame.h"
-#include "nsIHTMLDocument.h"
 #include "nsIHttpChannelInternal.h"
 #include "nsIIdleService.h"
 #include "nsIImageLoadingContent.h"
@@ -2656,14 +2655,13 @@ nsresult nsContentUtils::GenerateStateKey(nsIContent* aContent,
     return NS_OK;
   }
 
-  nsCOMPtr<nsIHTMLDocument> htmlDocument =
-      do_QueryInterface(aContent->GetUncomposedDoc());
+  RefPtr<Document> doc = aContent->GetUncomposedDoc();
 
   KeyAppendInt(partID, aKey);  // first append a partID
   bool generatedUniqueKey = false;
 
-  if (htmlDocument) {
-    nsHTMLDocument* htmlDoc = static_cast<nsHTMLDocument*>(htmlDocument.get());
+  if (doc && doc->IsHTMLOrXHTML()) {
+    nsHTMLDocument* htmlDoc = doc->AsHTMLDocument();
     RefPtr<nsContentList> htmlForms;
     RefPtr<nsContentList> htmlFormControls;
     htmlDoc->GetFormsAndFormControls(getter_AddRefs(htmlForms),
@@ -2708,7 +2706,7 @@ nsresult nsContentUtils::GenerateStateKey(nsIContent* aContent,
           // highly likely guess that the highest form parsed so far is the one.
           // This code should not be on trunk, only branch.
           //
-          index = htmlDocument->GetNumFormsSynchronous() - 1;
+          index = htmlDoc->GetNumFormsSynchronous() - 1;
         }
         if (index > -1) {
           KeyAppendInt(index, aKey);
@@ -4520,10 +4518,6 @@ already_AddRefed<DocumentFragment> nsContentUtils::CreateContextualFragment(
   // for compiling event handlers... so just bail out.
   RefPtr<Document> document = aContextNode->OwnerDoc();
   bool isHTML = document->IsHTMLDocument();
-#ifdef DEBUG
-  nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(document);
-  NS_ASSERTION(!isHTML || htmlDoc, "Should have HTMLDocument here!");
-#endif
 
   if (isHTML) {
     RefPtr<DocumentFragment> frag =
diff --git a/dom/base/nsCopySupport.cpp b/dom/base/nsCopySupport.cpp
index f3fab379d163..9d1d901e72b8 100644
--- a/dom/base/nsCopySupport.cpp
+++ b/dom/base/nsCopySupport.cpp
@@ -28,7 +28,7 @@
 
 #include "nsPIDOMWindow.h"
 #include "mozilla/dom/Document.h"
-#include "nsIHTMLDocument.h"
+#include "nsHTMLDocument.h"
 #include "nsGkAtoms.h"
 #include "nsIFrame.h"
 #include "nsIURI.h"
@@ -122,10 +122,9 @@ static nsresult EncodeForTextUnicode(nsIDocumentEncoder& aEncoder,
 
   if (!selForcedTextPlain && mimeType.EqualsLiteral(kTextMime)) {
     // SetSelection and EncodeToString use this case to signal that text/plain
-    // was forced because the document is either not an nsIHTMLDocument or it's
-    // XHTML.  We want to pretty print XHTML but not non-nsIHTMLDocuments.
-    nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(&aDocument);
-    if (!htmlDoc) {
+    // was forced because the document is either not an HTMLDocument or it's
+    // XHTML.  We want to pretty print XHTML but not non-HTMLDocuments.
+    if (!aDocument.IsHTMLOrXHTML()) {
       selForcedTextPlain = true;
     }
   }
@@ -562,10 +561,6 @@ static nsresult AppendDOMNode(nsITransferable* aTransferable,
   // Note that XHTML is not counted as HTML here, because we can't copy it
   // properly (all the copy code for non-plaintext assumes using HTML
   // serializers and parsers is OK, and those mess up XHTML).
-  DebugOnly<nsCOMPtr<nsIHTMLDocument>> htmlDoc =
-      nsCOMPtr<nsIHTMLDocument>(do_QueryInterface(document, &rv));
-  NS_ENSURE_SUCCESS(rv, NS_OK);
-
   NS_ENSURE_TRUE(document->IsHTMLDocument(), NS_OK);
 
   // init encoder with document and node
diff --git a/dom/base/nsDocumentEncoder.cpp b/dom/base/nsDocumentEncoder.cpp
index 232aed180008..cb40936b3271 100644
--- a/dom/base/nsDocumentEncoder.cpp
+++ b/dom/base/nsDocumentEncoder.cpp
@@ -17,13 +17,13 @@
 #include "nsIFactory.h"
 #include "nsISupports.h"
 #include "mozilla/dom/Document.h"
-#include "nsIHTMLDocument.h"
 #include "nsCOMPtr.h"
 #include "nsIContentSerializer.h"
 #include "mozilla/Encoding.h"
 #include "nsIOutputStream.h"
 #include "nsRange.h"
 #include "nsGkAtoms.h"
+#include "nsHTMLDocument.h"
 #include "nsIContent.h"
 #include "nsIScriptContext.h"
 #include "nsIScriptGlobalObject.h"
@@ -1347,8 +1347,7 @@ nsHTMLCopyEncoder::SetSelection(Selection* aSelection) {
   // XXX bug 1245883
 
   // also consider ourselves in a text widget if we can't find an html document
-  nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(mDocument);
-  if (!(htmlDoc && mDocument->IsHTMLDocument())) {
+  if (!(mDocument && mDocument->IsHTMLDocument())) {
     mIsTextWidget = true;
     mEncodingScope.mSelection = aSelection;
     // mMimeType is set to text/plain when encoding starts.
diff --git a/dom/base/nsFocusManager.cpp b/dom/base/nsFocusManager.cpp
index 82b7fcd61e28..a8ae3249121a 100644
--- a/dom/base/nsFocusManager.cpp
+++ b/dom/base/nsFocusManager.cpp
@@ -16,7 +16,6 @@
 #include "ContentParent.h"
 #include "nsPIDOMWindow.h"
 #include "nsIDOMChromeWindow.h"
-#include "nsIHTMLDocument.h"
 #include "nsIDocShell.h"
 #include "nsIDocShellTreeOwner.h"
 #include "nsIFormControl.h"
@@ -40,6 +39,7 @@
 #include "nsStyleCoord.h"
 #include "BrowserChild.h"
 #include "nsFrameLoader.h"
+#include "nsHTMLDocument.h"
 #include "nsNumberControlFrame.h"
 #include "nsNetUtil.h"
 #include "nsRange.h"
@@ -3839,8 +3839,7 @@ Element* nsFocusManager::GetRootForFocus(nsPIDOMWindowOuter* aWindow,
   }
 
   // Finally, check if this is a frameset
-  nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(aDocument);
-  if (htmlDoc) {
+  if (aDocument && aDocument->IsHTMLOrXHTML()) {
     Element* htmlChild = aDocument->GetHtmlChildElement(nsGkAtoms::frameset);
     if (htmlChild) {
       // In document navigation mode, return the frameset so that navigation
diff --git a/dom/html/HTMLFormElement.cpp b/dom/html/HTMLFormElement.cpp
index 9cf8a526a086..65e4fac99c64 100644
--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -18,8 +18,8 @@
 #include "mozilla/dom/HTMLFormControlsCollection.h"
 #include "mozilla/dom/HTMLFormElementBinding.h"
 #include "mozilla/Move.h"
-#include "nsIHTMLDocument.h"
 #include "nsGkAtoms.h"
+#include "nsHTMLDocument.h"
 #include "nsStyleConsts.h"
 #include "nsPresContext.h"
 #include "mozilla/dom/Document.h"
@@ -358,7 +358,7 @@ static void CollectOrphans(nsINode* aRemovalRoot,
 void HTMLFormElement::UnbindFromTree(bool aNullParent) {
   // Note, this is explicitly using uncomposed doc, since we count
   // only forms in document.
-  nsCOMPtr<nsIHTMLDocument> oldDocument = do_QueryInterface(GetUncomposedDoc());
+  RefPtr<Document> oldDocument = GetUncomposedDoc();
 
   // Mark all of our controls as maybe being orphans
   MarkOrphans(mControls->mElements);
@@ -396,8 +396,8 @@ void HTMLFormElement::UnbindFromTree(bool aNullParent) {
 #endif
   );
 
-  if (oldDocument) {
-    oldDocument->RemovedForm();
+  if (oldDocument && oldDocument->IsHTMLOrXHTML()) {
+    oldDocument->AsHTMLDocument()->RemovedForm();
   }
   ForgetCurrentSubmission();
 }
@@ -1483,8 +1483,7 @@ nsresult HTMLFormElement::GetActionURL(nsIURI** aActionURL,
 
   nsCOMPtr<nsIURI> actionURL;
   if (action.IsEmpty()) {
-    nsCOMPtr<nsIHTMLDocument> htmlDoc(do_QueryInterface(document));
-    if (!htmlDoc) {
+    if (!document->IsHTMLOrXHTML()) {
       // Must be a XML, XUL or other non-HTML document type
       // so do nothing.
       return NS_OK;
diff --git a/dom/html/moz.build b/dom/html/moz.build
index 7ac525b06f47..4fececd4eba8 100644
--- a/dom/html/moz.build
+++ b/dom/html/moz.build
@@ -39,7 +39,6 @@ EXPORTS += [
     'nsIFormControl.h',
     'nsIFormProcessor.h',
     'nsIHTMLCollection.h',
-    'nsIHTMLDocument.h',
     'nsIRadioGroupContainer.h',
     'nsIRadioVisitor.h',
     'nsITextControlElement.h',
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index ac272e68457f..ca1ef2ab5237 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -32,7 +32,6 @@
 #include "nsIDOMWindow.h"
 #include "nsMappedAttributes.h"
 #include "nsHTMLStyleSheet.h"
-#include "nsIHTMLDocument.h"
 #include "nsPIDOMWindow.h"
 #include "nsIURL.h"
 #include "nsEscape.h"
diff --git a/dom/html/nsHTMLContentSink.cpp b/dom/html/nsHTMLContentSink.cpp
index 41c39b9a3a0a..e9460e45c8d0 100644
--- a/dom/html/nsHTMLContentSink.cpp
+++ b/dom/html/nsHTMLContentSink.cpp
@@ -49,7 +49,7 @@
 #include "nsIDocShell.h"
 #include "mozilla/dom/Document.h"
 #include "nsStubDocumentObserver.h"
-#include "nsIHTMLDocument.h"
+#include "nsHTMLDocument.h"
 #include "nsICookieService.h"
 #include "nsTArray.h"
 #include "nsIScriptSecurityManager.h"
@@ -137,7 +137,7 @@ class HTMLContentSink : public nsContentSink, public nsIHTMLContentSink {
  protected:
   virtual ~HTMLContentSink();
 
-  nsCOMPtr<nsIHTMLDocument> mHTMLDocument;
+  RefPtr<nsHTMLDocument> mHTMLDocument;
 
   // The maximum length of a text run
   int32_t mMaxTextRun;
@@ -612,7 +612,7 @@ nsresult HTMLContentSink::Init(Document* aDoc, nsIURI* aURI,
 
   aDoc->AddObserver(this);
   mIsDocumentObserver = true;
-  mHTMLDocument = do_QueryInterface(aDoc);
+  mHTMLDocument = aDoc->AsHTMLDocument();
 
   NS_ASSERTION(mDocShell, "oops no docshell!");
 
@@ -960,6 +960,6 @@ void HTMLContentSink::ContinueInterruptedParsingAsync() {
       "HTMLContentSink::ContinueInterruptedParsingIfEnabled", this,
       &HTMLContentSink::ContinueInterruptedParsingIfEnabled);
 
-  nsCOMPtr<Document> doc = do_QueryInterface(mHTMLDocument);
+  RefPtr<Document> doc = mHTMLDocument;
   doc->Dispatch(mozilla::TaskCategory::Other, ev.forget());
 }
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index ea8df551c317..e8bcd56d8783 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -160,8 +160,7 @@ nsHTMLDocument::~nsHTMLDocument() {}
 
 NS_IMPL_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, Document, mAll)
 
-NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(nsHTMLDocument, Document,
-                                             nsIHTMLDocument)
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(nsHTMLDocument, Document)
 
 JSObject* nsHTMLDocument::WrapNode(JSContext* aCx,
                                    JS::Handle<JSObject*> aGivenProto) {
diff --git a/dom/html/nsHTMLDocument.h b/dom/html/nsHTMLDocument.h
index 9423015f9613..125213a8f398 100644
--- a/dom/html/nsHTMLDocument.h
+++ b/dom/html/nsHTMLDocument.h
@@ -9,7 +9,6 @@
 #include "mozilla/Attributes.h"
 #include "nsContentList.h"
 #include "mozilla/dom/Document.h"
-#include "nsIHTMLDocument.h"
 #include "nsIHTMLCollection.h"
 #include "nsIScriptElement.h"
 #include "nsTArray.h"
@@ -35,7 +34,7 @@ class WindowProxyHolder;
 }  // namespace dom
 }  // namespace mozilla
 
-class nsHTMLDocument : public mozilla::dom::Document, public nsIHTMLDocument {
+class nsHTMLDocument : public mozilla::dom::Document {
  protected:
   typedef mozilla::net::ReferrerPolicy ReferrerPolicy;
   typedef mozilla::dom::Document Document;
@@ -70,7 +69,6 @@ class nsHTMLDocument : public mozilla::dom::Document, public nsIHTMLDocument {
   virtual bool UseWidthDeviceWidthFallbackViewport() const override;
 
  public:
-  // nsIHTMLDocument
   virtual Element* GetUnfocusedKeyEventTarget() override;
 
   nsContentList* GetExistingForms() const { return mForms; }
@@ -82,9 +80,23 @@ class nsHTMLDocument : public mozilla::dom::Document, public nsIHTMLDocument {
                    JS::MutableHandle<JS::Value> aRetval,
                    mozilla::ErrorResult& aError);
 
-  virtual void AddedForm() override;
-  virtual void RemovedForm() override;
-  virtual int32_t GetNumFormsSynchronous() override;
+  /**
+   * Called when form->BindToTree() is called so that document knows
+   * immediately when a form is added
+   */
+  void AddedForm();
+  /**
+   * Called when form->SetDocument() is called so that document knows
+   * immediately when a form is removed
+   */
+  void RemovedForm();
+  /**
+   * Called to get a better count of forms than document.forms can provide
+   * without calling FlushPendingNotifications (bug 138892).
+   */
+  // XXXbz is this still needed now that we can flush just content,
+  // not the rest?
+  int32_t GetNumFormsSynchronous();
   void SetIsXHTML(bool aXHTML) { mType = (aXHTML ? eXHTML : eHTML); }
 
   virtual nsresult Clone(mozilla::dom::NodeInfo*,
@@ -217,8 +229,4 @@ inline nsHTMLDocument* Document::AsHTMLDocument() {
 }  // namespace dom
 }  // namespace mozilla
 
-#define NS_HTML_DOCUMENT_INTERFACE_TABLE_BEGIN(_class) \
-  NS_DOCUMENT_INTERFACE_TABLE_BEGIN(_class)            \
-  NS_INTERFACE_TABLE_ENTRY(_class, nsIHTMLDocument)
-
 #endif /* nsHTMLDocument_h___ */
diff --git a/dom/html/nsIHTMLDocument.h b/dom/html/nsIHTMLDocument.h
deleted file mode 100644
index 0e54a2fa8681..000000000000
--- a/dom/html/nsIHTMLDocument.h
+++ /dev/null
@@ -1,51 +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 nsIHTMLDocument_h
-#define nsIHTMLDocument_h
-
-#include "nsISupports.h"
-#include "nsCompatibility.h"
-
-class nsIContent;
-class nsContentList;
-
-#define NS_IHTMLDOCUMENT_IID                         \
-  {                                                  \
-    0xcf814492, 0x303c, 0x4718, {                    \
-      0x9a, 0x3e, 0x39, 0xbc, 0xd5, 0x2c, 0x10, 0xdb \
-    }                                                \
-  }
-
-/**
- * HTML document extensions to Document.
- */
-class nsIHTMLDocument : public nsISupports {
- public:
-  NS_DECLARE_STATIC_IID_ACCESSOR(NS_IHTMLDOCUMENT_IID)
-
-  /**
-   * Called when form->BindToTree() is called so that document knows
-   * immediately when a form is added
-   */
-  virtual void AddedForm() = 0;
-  /**
-   * Called when form->SetDocument() is called so that document knows
-   * immediately when a form is removed
-   */
-  virtual void RemovedForm() = 0;
-  /**
-   * Called to get a better count of forms than document.forms can provide
-   * without calling FlushPendingNotifications (bug 138892).
-   */
-  // XXXbz is this still needed now that we can flush just content,
-  // not the rest?
-  virtual int32_t GetNumFormsSynchronous() = 0;
-};
-
-NS_DEFINE_STATIC_IID_ACCESSOR(nsIHTMLDocument, NS_IHTMLDOCUMENT_IID)
-
-#endif /* nsIHTMLDocument_h */
diff --git a/dom/prototype/PrototypeDocumentContentSink.cpp b/dom/prototype/PrototypeDocumentContentSink.cpp
index 2c5741f604c1..dc7e00c115ab 100644
--- a/dom/prototype/PrototypeDocumentContentSink.cpp
+++ b/dom/prototype/PrototypeDocumentContentSink.cpp
@@ -45,7 +45,6 @@
 #include "nsError.h"
 #include "nsNodeUtils.h"
 #include "nsIScriptGlobalObject.h"
-#include "nsIHTMLDocument.h"
 #include "mozAutoDocUpdate.h"
 #include "nsMimeTypes.h"
 #include "nsHtml5SVGLoadDispatcher.h"
diff --git a/dom/xhr/XMLHttpRequestMainThread.cpp b/dom/xhr/XMLHttpRequestMainThread.cpp
index ba070b657cde..28a9a0f72bde 100644
--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -67,7 +67,6 @@
 #include "nsContentUtils.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsError.h"
-#include "nsIHTMLDocument.h"
 #include "nsIStorageStream.h"
 #include "nsIPromptFactory.h"
 #include "nsIWindowWatcher.h"
diff --git a/dom/xml/nsXMLContentSink.cpp b/dom/xml/nsXMLContentSink.cpp
index 8ad71fcb360a..57a05ea39a25 100644
--- a/dom/xml/nsXMLContentSink.cpp
+++ b/dom/xml/nsXMLContentSink.cpp
@@ -45,7 +45,6 @@
 #include "nsError.h"
 #include "nsNodeUtils.h"
 #include "nsIScriptGlobalObject.h"
-#include "nsIHTMLDocument.h"
 #include "mozAutoDocUpdate.h"
 #include "nsMimeTypes.h"
 #include "nsHtml5SVGLoadDispatcher.h"
diff --git a/dom/xslt/xslt/txMozillaXMLOutput.cpp b/dom/xslt/xslt/txMozillaXMLOutput.cpp
index dbb703e34978..80f79adfb13d 100644
--- a/dom/xslt/xslt/txMozillaXMLOutput.cpp
+++ b/dom/xslt/xslt/txMozillaXMLOutput.cpp
@@ -20,7 +20,6 @@
 #include "nsNameSpaceManager.h"
 #include "txStringUtils.h"
 #include "txURIUtils.h"
-#include "nsIHTMLDocument.h"
 #include "nsIStyleSheetLinkingElement.h"
 #include "nsIDocumentTransformer.h"
 #include "mozilla/StyleSheetInlines.h"
diff --git a/editor/composer/nsEditingSession.cpp b/editor/composer/nsEditingSession.cpp
index 4c2051561220..aeed721ea5ea 100644
--- a/editor/composer/nsEditingSession.cpp
+++ b/editor/composer/nsEditingSession.cpp
@@ -30,7 +30,6 @@
 #include "mozilla/dom/Document.h"  // for Document
 #include "nsIDocumentStateListener.h"
 #include "nsIEditor.h"                   // for nsIEditor
-#include "nsIHTMLDocument.h"             // for nsIHTMLDocument, etc
 #include "nsIInterfaceRequestorUtils.h"  // for do_GetInterface
 #include "nsIPlaintextEditor.h"          // for nsIPlaintextEditor, etc
 #include "nsIRefreshURI.h"               // for nsIRefreshURI
diff --git a/editor/libeditor/EditorBase.cpp b/editor/libeditor/EditorBase.cpp
index 38c8de17044f..6873e5b446d5 100644
--- a/editor/libeditor/EditorBase.cpp
+++ b/editor/libeditor/EditorBase.cpp
@@ -83,7 +83,6 @@
 #include "nsIEditorObserver.h"         // for nsIEditorObserver
 #include "nsIEditorSpellCheck.h"       // for nsIEditorSpellCheck
 #include "nsIFrame.h"                  // for nsIFrame
-#include "nsIHTMLDocument.h"           // for nsIHTMLDocument
 #include "nsIInlineSpellChecker.h"     // for nsIInlineSpellChecker, etc.
 #include "nsNameSpaceManager.h"        // for kNameSpaceID_None, etc.
 #include "nsINode.h"                   // for nsINode, etc.
diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index c3a2f9962a9a..5b6d5e4dccec 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -31,8 +31,8 @@
 #include "nsNodeInfoManager.h"
 #include "nsContentCreatorFunctions.h"
 #include "mozilla/PresState.h"
-#include "nsIHTMLDocument.h"
 #include "nsContentUtils.h"
+#include "nsHTMLDocument.h"
 #include "nsLayoutUtils.h"
 #include "nsBidiPresUtils.h"
 #include "nsBidiUtils.h"
@@ -5331,8 +5331,7 @@ nsIFrame* ScrollFrameHelper::GetFrameForDir() const {
     Element* root = document->GetRootElement();
 
     // But for HTML and XHTML we want the body element.
-    nsCOMPtr<nsIHTMLDocument> htmlDoc = do_QueryInterface(document);
-    if (htmlDoc) {
+    if (document && document->IsHTMLOrXHTML()) {
       Element* bodyElement = document->GetBodyElement();
       if (bodyElement) {
         root = bodyElement;  // we can trust the document to hold on to it
diff --git a/parser/html/nsHtml5DocumentBuilder.cpp b/parser/html/nsHtml5DocumentBuilder.cpp
index 94470092238f..9400aab4fe91 100644
--- a/parser/html/nsHtml5DocumentBuilder.cpp
+++ b/parser/html/nsHtml5DocumentBuilder.cpp
@@ -7,7 +7,6 @@
 #include "nsHtml5DocumentBuilder.h"
 
 #include "mozilla/dom/ScriptLoader.h"
-#include "nsIHTMLDocument.h"
 #include "nsIStyleSheetLinkingElement.h"
 #include "nsNameSpaceManager.h"
 #include "nsStyleLinkElement.h"
diff --git a/parser/html/nsHtml5OplessBuilder.cpp b/parser/html/nsHtml5OplessBuilder.cpp
index a6058764f96d..60bbe72928d6 100644
--- a/parser/html/nsHtml5OplessBuilder.cpp
+++ b/parser/html/nsHtml5OplessBuilder.cpp
@@ -9,7 +9,6 @@
 #include "mozilla/css/Loader.h"
 #include "mozilla/dom/ScriptLoader.h"
 #include "nsIDocShell.h"
-#include "nsIHTMLDocument.h"
 
 nsHtml5OplessBuilder::nsHtml5OplessBuilder() : nsHtml5DocumentBuilder(true) {}
 
diff --git a/parser/html/nsHtml5TreeOpExecutor.cpp b/parser/html/nsHtml5TreeOpExecutor.cpp
index 93763dd78b83..d543c9a57e8e 100644
--- a/parser/html/nsHtml5TreeOpExecutor.cpp
+++ b/parser/html/nsHtml5TreeOpExecutor.cpp
@@ -29,7 +29,6 @@
 #include "nsIContentViewer.h"
 #include "nsIDocShell.h"
 #include "nsIDocShellTreeItem.h"
-#include "nsIHTMLDocument.h"
 #include "nsINestedURI.h"
 #include "nsIScriptContext.h"
 #include "nsIScriptError.h"
diff --git a/parser/html/nsHtml5TreeOperation.cpp b/parser/html/nsHtml5TreeOperation.cpp
index 6cd8f0cf6cc4..af7ee7ef53e8 100644
--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -28,7 +28,6 @@
 #include "nsIDTD.h"
 #include "nsIFormControl.h"
 #include "nsIFormProcessor.h"
-#include "nsIHTMLDocument.h"
 #include "nsIMutationObserver.h"
 #include "nsINode.h"
 #include "nsIObserverService.h"