incubator-pagespeed-ngx/install/mod_pagespeed_example/proxy_external_resource.html

<!-- This example demonstrates proxying & optimizing a resource served
     on another domain -->

<!--
   Example image proxied and optimized.  This image is a 1x1
   transparent image stored on Google's static content server for the
   benefit of lazyload_images.
-->
<img src="http://www.gstatic.com/psa/static/1.gif"/>