--- a/dom/interfaces/css/moz.build
+++ b/dom/interfaces/css/moz.build
@@ -10,16 +10,15 @@ with Files("**"):
 XPIDL_SOURCES += [
     'nsIDOMCounter.idl',
     'nsIDOMCSSPrimitiveValue.idl',
     'nsIDOMCSSRule.idl',
     'nsIDOMCSSRuleList.idl',
     'nsIDOMCSSStyleDeclaration.idl',
     'nsIDOMCSSStyleRule.idl',
     'nsIDOMCSSStyleSheet.idl',
-    'nsIDOMCSSUnknownRule.idl',
     'nsIDOMCSSValue.idl',
     'nsIDOMCSSValueList.idl',
     'nsIDOMRect.idl',
 ]
 
 XPIDL_MODULE = 'dom_css'
 

deleted file mode 100644
--- a/dom/interfaces/css/nsIDOMCSSUnknownRule.idl
+++ /dev/null
@@ -1,11 +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 "nsISupports.idl"
-
-[scriptable, uuid(98f4c27b-fb35-4355-8fd9-546c4697d71a)]
-interface nsIDOMCSSUnknownRule : nsISupports
-{
-};