&gt; what&#x27;s &quot;third-party&quot;<p>That&#x27;s already defined by the same-origin policy.<p>&gt; a common optimization technique<p>Optimization should never happen at the expense of security. (and yes, 3rd party scripts are a security risk)<p>&gt; increase the number of parallel connections.<p>I believe that trick is to save bandwidth by not sending the (large) cookies that would normally be sent to the normal domain.