IDL: make optional arguments (without default) explicit sometimes

Source/core/dom/DOMImplementation.idl

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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,
@@ skipping 17 matching lines @@
     [ImplementedAs=hasFeatureForBindings, MeasureAs=DOMImplementationHasFeature]
     boolean hasFeature(DOMString feature, DOMString? version);
 
     // DOM Level 2
 
     [RaisesException] DocumentType createDocumentType(DOMString qualifiedName,
                                                       DOMString publicId,
                                                       DOMString systemId);
     [RaisesException] XMLDocument createDocument(DOMString? namespaceURI,
                                                  DOMString? qualifiedName,
-                                                 [Default=Undefined] optional DocumentType doctype);
+                                                 optional DocumentType doctype);
 
     // HTMLDOMImplementation interface from DOM Level 2 HTML
 
     HTMLDocument createHTMLDocument(optional DOMString title = null);
 };