install/mod_pagespeed_example/responsive_images_iframe.html

<style>
*{margin: 0;padding: 0}
div{position:absolute};
</style>
<div></div>
<img width="500" height="500"
     src="images/Beach.jpg">
<script>
window.setInterval(function() {
  var img = document.getElementsByTagName("img")[0];
  var div = document.getElementsByTagName("div")[0];
  var a = "PageSpeedFilters=responsive_images, resize_images";
  if (window.devicePixelRatio) {
    a += "<br>dpr: " + window.devicePixelRatio.toFixed(2) + "x";
  }
  if (img.currentSrc) {
    a += "<br>" + img.currentSrc.replace(/^.*[\/]/, '');
  }
  div.innerHTML = a;
}, 100);
</script>