Sync the HTMLCollection interfaces with the specs

Source/core/html/HTMLCollection.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2014 Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
+// https://dom.spec.whatwg.org/#interface-htmlcollection
+
 [
     DependentLifetime,
     SetWrapperReferenceFrom=ownerNode,
     WillBeGarbageCollected,
 ] interface HTMLCollection {
     readonly attribute unsigned long length;
-    getter Element item([Default=Undefined] optional unsigned long index);
-    getter Element namedItem(DOMString name);
+    // FIXME: The index argument should not be optional.
+    getter Element? item([Default=Undefined] optional unsigned long index);
+    getter Element? namedItem(DOMString name);
 };
-