Support basic immutable CSSOM

components/script/dom/webidls/CSSNamespaceRule.webidl

@@ -0,0 +1,10 @@
+/* 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/. */
+
+// https://drafts.csswg.org/cssom/#the-cssnamespacerule-interface
+[Exposed=(Window,Worker)]
+interface CSSNamespaceRule : CSSRule {
+  // readonly attribute DOMString namespaceURI;
+  // readonly attribute DOMString prefix;
+};