IDL: make optional arguments (without default) explicit sometimes

Source/core/css/CSSKeyframesRule.idl

 /*
  * Copyright (C) 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 14 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // Introduced in DOM Level ?:
 interface CSSKeyframesRule : CSSRule {
     attribute DOMString name;
     readonly attribute CSSRuleList cssRules;
 
     [ImplementedAs=item, NotEnumerable] getter CSSKeyframeRule (unsigned long index);
-    void insertRule([Default=Undefined] optional DOMString rule);
-    void deleteRule([Default=Undefined] optional DOMString key);
-    CSSKeyframeRule findRule([Default=Undefined] optional DOMString key);
+    void insertRule(optional DOMString rule);
+    void deleteRule(optional DOMString key);
+    CSSKeyframeRule findRule(optional DOMString key);
 };