Tidy finds WebIDLs with no spec.

Closes #6689

components/script/dom/webidls/HTMLCollection.webidl

@@ -3,6 +3,8 @@
  * 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/. */
 
+// http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-75708506
+
 interface HTMLCollection {
   readonly attribute unsigned long length;
   getter Element? item(unsigned long index);