[cssom-1] Align with Web IDL specification (w3c#5444)

Author: autokagami

cssom-1/Overview.bs

@@ -510,8 +510,8 @@ interface MediaList {
   stringifier attribute [LegacyNullToEmptyString] CSSOMString mediaText;
   readonly attribute unsigned long length;
   getter CSSOMString? item(unsigned long index);
-  void appendMedium(CSSOMString medium);
-  void deleteMedium(CSSOMString medium);
+  undefined appendMedium(CSSOMString medium);
+  undefined deleteMedium(CSSOMString medium);
 };
 </pre>
 
@@ -910,7 +910,7 @@ interface CSSStyleSheet : StyleSheet {
   readonly attribute CSSRule? ownerRule;
   [SameObject] readonly attribute CSSRuleList cssRules;
   unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
-  void deleteRule(unsigned long index);
+  undefined deleteRule(unsigned long index);
 };
 </pre>
 
@@ -957,7 +957,7 @@ The <dfn method for=CSSStyleSheet>deleteRule(<var>index</var>)</dfn> method must
 partial interface CSSStyleSheet {
   [SameObject] readonly attribute CSSRuleList rules;
   long addRule(optional DOMString selector = "undefined", optional DOMString style = "undefined", optional unsigned long index);
-  void removeRule(optional unsigned long index = 0);
+  undefined removeRule(optional unsigned long index = 0);
 };
 </pre>
 
@@ -1859,7 +1859,7 @@ The <code>CSSGroupingRule</code> interface represents an at-rule that contains o
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
   unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
-  void deleteRule(unsigned long index);
+  undefined deleteRule(unsigned long index);
 };
 </pre>
 
@@ -2153,7 +2153,7 @@ interface CSSStyleDeclaration {
   getter CSSOMString item(unsigned long index);
   CSSOMString getPropertyValue(CSSOMString property);
   CSSOMString getPropertyPriority(CSSOMString property);
-  [CEReactions] void setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value, optional [LegacyNullToEmptyString] CSSOMString priority = "");
+  [CEReactions] undefined setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value, optional [LegacyNullToEmptyString] CSSOMString priority = "");
   [CEReactions] CSSOMString removeProperty(CSSOMString property);
   readonly attribute CSSRule? parentRule;
   [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString cssFloat;