="ie_legacy_event_support" style="display: none">
    Extending event objects (click to test)
  </p>
  
  <script type="text/javascript">
    Event.observe(window, 'load', function() {
      // Ensures we can manually extend events in IE's legacy event system.
      // IE9 supports both the legacy system and DOM L2 events, so we have to
      // inspect an event object at runtime to determine if it needs to be
      // extended.      
      if (!window.attachEvent) return;
      
      function mouseButton(