diff --git a/dom/interfaces/css/nsIDOMCSSGroupingRule.idl b/dom/interfaces/css/nsIDOMCSSGroupingRule.idl
index 80f072d16..f3580aeec 100644
--- a/dom/interfaces/css/nsIDOMCSSGroupingRule.idl
+++ b/dom/interfaces/css/nsIDOMCSSGroupingRule.idl
@@ -14,7 +14,7 @@ interface nsIDOMCSSGroupingRule : nsIDOMCSSRule
   readonly attribute nsIDOMCSSRuleList cssRules;
 
   unsigned long      insertRule(in DOMString rule,
-                                in unsigned long index)
+                                [optional] in unsigned long index)
                                         raises(DOMException);
   void               deleteRule(in unsigned long index)
                                         raises(DOMException);
diff --git a/dom/interfaces/css/nsIDOMCSSStyleSheet.idl b/dom/interfaces/css/nsIDOMCSSStyleSheet.idl
index b9e13134e..cefe4f704 100644
--- a/dom/interfaces/css/nsIDOMCSSStyleSheet.idl
+++ b/dom/interfaces/css/nsIDOMCSSStyleSheet.idl
@@ -20,7 +20,7 @@ interface nsIDOMCSSStyleSheet : nsIDOMStyleSheet
   readonly attribute nsIDOMCSSRuleList cssRules;
 
   unsigned long      insertRule(in DOMString rule, 
-                                in unsigned long index)
+                                [optional] in unsigned long index)
                                         raises(DOMException);
   void               deleteRule(in unsigned long index)
                                         raises(DOMException);
diff --git a/dom/webidl/CSSStyleSheet.webidl b/dom/webidl/CSSStyleSheet.webidl
index 1a368aec2..dc88888c3 100644
--- a/dom/webidl/CSSStyleSheet.webidl
+++ b/dom/webidl/CSSStyleSheet.webidl
@@ -15,7 +15,7 @@ interface CSSStyleSheet : StyleSheet {
   [Throws]
   readonly attribute CSSRuleList cssRules;
   [Throws]
-  unsigned long insertRule(DOMString rule, unsigned long index);
+  unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws]
   void deleteRule(unsigned long index);
 };