Here is an example on how to feature detect an Asynchronous event using a Synchronous event. Being able to capture 'load' events of each resource in a web page is a very interesting feature. All resources could be logged, or modified with just one 'load'' capturing event handler. This works for external link elements (CSS), for iframes, images, …

has_load_capture_support.js

/*
 *
 * Feature detect support for capturing load events
 *
 * Author: Diego Perini
 * Updated: 2013/06/25
 *
 * the feature test can be performed before
 * both 'load' and 'DOMContentLoaded' events
 *
 * a synchronous event is triggered to test
 * browsers handling of the dispatched event
 *
 * IE6/7/8 have no capturing event support at all
 * all other browsers should support this feature
 *
 * document.addEventListener('load', handler, true);
 *
 */

function has_load_capture_support() {
  var d = document, event,
  root = d.documentElement,
  done = false, support = false;
  if (d.addEventListener) {
    event = d.createEvent('Event');
    function handler(event) {
      if (event.target == root && event.eventPhase == 1) {
        support = true;
        d.removeEventListener('load', handler, true);
      }
    }
    d.addEventListener('load', handler, true);
    event.initEvent('load', true, true);
    done = root.dispatchEvent(event);
  }
  return done && support;
}