[cssom] Don't serialize shorthand if importance differs

Author: upsuper

cssom/Overview.bs

@@ -1980,7 +1980,9 @@ The <dfn method for=CSSStyleDeclaration>getPropertyValue(<var>property</var>)</d
        <li>If <var>declaration</var> is null, return the empty string and terminate these steps.
        <li>Append the <var>declaration</var> to <var>list</var>.
       </ol>
-     <li>Return the <a lt="serialize a CSS value">serialization</a> of <var>list</var> and terminate these steps.
+     <li>If <a lt="important flag" for="CSS declaration">important flags</a> of all declarations in <var>list</var> are same,
+     return the <a lt="serialize a CSS value">serialization</a> of <var>list</var> and terminate these steps.
+     <li>Return the empty string.
     </ol>
   </ol>
  <li>If <var>property</var> is a <a>case-sensitive</a>