The spec on this is at <a href="http://url.spec.whatwg.org/#dom-url-host" rel="nofollow">http://url.spec.whatwg.org/#dom-url-host</a> and says:<p><pre><code> If url is null, return the empty string.
If port is the empty string, return host, serialized.
Return host, serialized, ":", and port concatenated.
</code></pre>
So what is port in this case? That's defined at <a href="http://url.spec.whatwg.org/#port-state" rel="nofollow">http://url.spec.whatwg.org/#port-state</a> and the key step is:<p><pre><code> 2. If buffer is equal to url's scheme's default port,
set buffer to the empty string.
</code></pre>
So IE is wrong and the MDN documentation is misleading. I've fixed the latter; can't do much about IE. ;)