[css-typed-om-1] Align with Web IDL specification (w3c#1006)

Author: autokagami

css-typed-om/Overview.bs

@@ -310,10 +310,10 @@ interface StylePropertyMapReadOnly {
 
 [Exposed=Window]
 interface StylePropertyMap : StylePropertyMapReadOnly {
-    void set(USVString property, (CSSStyleValue or USVString)... values);
-    void append(USVString property, (CSSStyleValue or USVString)... values);
-    void delete(USVString property);
-    void clear();
+    undefined set(USVString property, (CSSStyleValue or USVString)... values);
+    undefined append(USVString property, (CSSStyleValue or USVString)... values);
+    undefined delete(USVString property);
+    undefined clear();
 };
 </xmp>