[css-animations-1] Align with Web IDL specification

Author: autokagami

css-animations-1/Overview.bs

@@ -1118,8 +1118,8 @@ IDL Definition</h4>
 	           attribute CSSOMString name;
 	  readonly attribute CSSRuleList cssRules;
 
-	  void             appendRule(CSSOMString rule);
-	  void             deleteRule(CSSOMString select);
+	  undefined             appendRule(CSSOMString rule);
+	  undefined             deleteRule(CSSOMString select);
 	  CSSKeyframeRule? findRule(CSSOMString select);
 	};
 	</pre>