diff --git a/content/html/content/src/HTMLUnknownElement.cpp b/content/html/content/src/HTMLUnknownElement.cpp
index 40722bb2f1b7..c20d7aaff2a8 100644
--- a/content/html/content/src/HTMLUnknownElement.cpp
+++ b/content/html/content/src/HTMLUnknownElement.cpp
@@ -4,7 +4,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsDocument.h"
-#include "HTMLUnknownElement.h"
+#include "mozilla/dom/HTMLUnknownElement.h"
 #include "mozilla/dom/HTMLElementBinding.h"
 
 NS_IMPL_NS_NEW_HTML_ELEMENT(Unknown)
@@ -35,11 +35,8 @@ HTMLUnknownElement::WrapNode(JSContext *aCx, JS::Handle<JSObject*> aScope)
 }
 
 // QueryInterface implementation for HTMLUnknownElement
-NS_INTERFACE_TABLE_HEAD(HTMLUnknownElement)
+NS_INTERFACE_MAP_BEGIN(HTMLUnknownElement)
   NS_HTML_CONTENT_INTERFACES(nsGenericHTMLElement)
-  NS_INTERFACE_TABLE_INHERITED1(HTMLUnknownElement,
-                                nsIDOMHTMLUnknownElement)
-  NS_INTERFACE_TABLE_TO_MAP_SEGUE
 NS_ELEMENT_INTERFACE_MAP_END
 
 NS_IMPL_ELEMENT_CLONE(HTMLUnknownElement)
diff --git a/content/html/content/src/HTMLUnknownElement.h b/content/html/content/src/HTMLUnknownElement.h
index 7cd566ce65a3..fa117c22edcb 100644
--- a/content/html/content/src/HTMLUnknownElement.h
+++ b/content/html/content/src/HTMLUnknownElement.h
@@ -2,18 +2,17 @@
 /* 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 HTMLUnknownElement_h___
-#define HTMLUnknownElement_h___
+#ifndef mozilla_dom_HTMLUnknownElement_h
+#define mozilla_dom_HTMLUnknownElement_h
 
 #include "mozilla/Attributes.h"
 #include "nsGenericHTMLElement.h"
-#include "nsIDOMHTMLUnknownElement.h"
 
 namespace mozilla {
 namespace dom {
 
 class HTMLUnknownElement MOZ_FINAL : public nsGenericHTMLElement
-                                   , public nsIDOMHTMLUnknownElement
+                                   , public nsIDOMHTMLElement
 {
 public:
   HTMLUnknownElement(already_AddRefed<nsINodeInfo> aNodeInfo)
@@ -48,4 +47,4 @@ protected:
 } // namespace dom
 } // namespace mozilla
 
-#endif /* HTMLUnknownElement_h___ */
+#endif /* mozilla_dom_HTMLUnknownElement_h */
diff --git a/content/html/content/test/test_bug389797.html b/content/html/content/test/test_bug389797.html
index 42a5ee12f338..ce5bb19807e1 100644
--- a/content/html/content/test/test_bug389797.html
+++ b/content/html/content/test/test_bug389797.html
@@ -244,13 +244,9 @@ for (var tag of allTags) {
   if (classInfoString != 'HTMLUnknownElement') {
     is(node instanceof HTMLUnknownElement, false,
        tagName(tag) + " is an instance of HTMLUnknownElement");
-    is(node instanceof SpecialPowers.Ci.nsIDOMHTMLUnknownElement, false,
-       tagName(tag) + " is an instance of nsIDOMHTMLUnknownElement");
   } else {
     is(node instanceof HTMLUnknownElement, true,
        tagName(tag) + " is an instance of HTMLUnknownElement");
-    is(node instanceof SpecialPowers.Ci.nsIDOMHTMLUnknownElement, true,
-       tagName(tag) + " is an instance of nsIDOMHTMLUnknownElement");
   }
 
   // Check that each node QIs to all the things we expect it to QI to
diff --git a/dom/interfaces/html/moz.build b/dom/interfaces/html/moz.build
index e576cf9755fe..864d29b9d31a 100644
--- a/dom/interfaces/html/moz.build
+++ b/dom/interfaces/html/moz.build
@@ -65,7 +65,6 @@ XPIDL_SOURCES += [
     'nsIDOMHTMLTextAreaElement.idl',
     'nsIDOMHTMLTitleElement.idl',
     'nsIDOMHTMLUListElement.idl',
-    'nsIDOMHTMLUnknownElement.idl',
     'nsIDOMHTMLVideoElement.idl',
     'nsIDOMMediaError.idl',
     'nsIDOMMozBrowserFrame.idl',
diff --git a/dom/interfaces/html/nsIDOMHTMLUnknownElement.idl b/dom/interfaces/html/nsIDOMHTMLUnknownElement.idl
deleted file mode 100644
index 9a53b81bd820..000000000000
--- a/dom/interfaces/html/nsIDOMHTMLUnknownElement.idl
+++ /dev/null
@@ -1,17 +0,0 @@
-/* -*- 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/. */
-
-#include "nsIDOMHTMLElement.idl"
-
-/**
- * The nsIDOMHTMLUnknownElement interface is the interface to an unknown HTML
- * element.
- *
- * @see <http://www.whatwg.org/html/#htmlunknownelement>
- */
-[scriptable, uuid(aa044a2d-4e9b-4fc5-8ad2-666da9062b36)]
-interface nsIDOMHTMLUnknownElement : nsIDOMHTMLElement
-{
-};