[css-fonts-4] Align with Web IDL specification

Author: autokagami

css-fonts-4/Overview.bs

@@ -6114,9 +6114,10 @@ interface CSSFontFeatureValuesRule : CSSRule {
 [Exposed=Window]
 interface CSSFontFeatureValuesMap {
   maplike<CSSOMString, sequence<unsigned long>>;
-  void set(CSSOMString featureValueName,
+  undefined set(CSSOMString featureValueName,
            (unsigned long or sequence<unsigned long>) values);
-};</pre>
+};
+</pre>
 
 <dl class='idl-attributes'>
 <dt><var>fontFamily</var> of type <code>CSSOMString</code>