How to manually show tab loading indicator via javascript?

I'm talking about an icon that is displayed on a tab during page loading.

Chrome:

Firefox (with TreeTab plugin):

You get the idea. I want to make it seem like the page is loading, when it's already loaded. Some event fires is javascript and then the tab looks like it's being loaded. Is there a way to do that?

One way I can think of is to replace a favicon with a spinner, but I'm not sure if it's possible to change on the fly and even if it is, it would be a hassle to make it cross-browser.

I don't think it is a good idea to do it, you'll make your users do a lot of useless requests, and this kills trees : /

IMO, it's better to do all you have in the page itself, and let the browser's UI do his own stuff.

But since I liked the challenge, here is one hacky way :

Loading an iframe will trigger this icon in both chrome and Firefox^[1], so you could ,

• append an iframe in the document,
• set its src to some huge document,
• onload of the iframe, set it again with a ? cache hack,
• regularly check if the duration has elapsed so you can remove the iframe's src.

[1] It seems that Firefox does trigger the icon only if it was triggered when the document was still loading.

In code :

// how to use : 
showTabLoader(25000);

// takes duration in ms as only parameter
function showTabLoader(duration) {

  if (!duration) return;

  var now = performance.now();
  // To avoid flickering, you need some big document
  // Please change this url in your script, wikimedia may not be happy with us.
  var url = 'https://upload.wikimedia.org/wikipedia/commons/3/35/Viborg_Katedralskole_Symmetrical.jpg';

  var iframe = document.createElement('iframe');
  iframe.style.display = 'none';
  document.body.appendChild(iframe);
  iframe.onload = function() {
    if (performance.now() - now < +duration) {
      this.src = url + '?' + Math.random();
    }
  };
  var check = function(time) {
    if (time - now > +duration) {
      iframe.src = '';
      iframe.parentNode.removeChild(iframe);
      return;
    }
    requestAnimationFrame(check);
  }
  requestAnimationFrame(check);
  iframe.src = url;

}