url.port = '' sets the port number to null.

It was previously (per spec) a no-op.

This matches Firefox. (Chromium sets the port to zero.)
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4054

url.bs

@@ -1595,6 +1595,9 @@ optionally with an <a>encoding</a>
          <li><p>Set <var>buffer</var> to the empty string.
         </ol>
 
+       <li><p>If <var>state override</var> is given and <var>buffer</var> is the empty string,
+       Set <var>url</var>'s <a for=url>port</a> to null.
+
        <li><p>If <var>state override</var> is given, terminate this algorithm.
 
        <li><p>Set <var>state</var> to <a>path start state</a>, and decrease