async_defer_script.js

// ==UserScript==
// @name           defer/async for Opera
// @namespace      http://d.hatena.ne.jp/edvakf/
// @license        Public Domain
// ==/UserScript==

(function(window, document, opera) {
  function load_script(script, attr) {
    return function() {
      var script2 = script.cloneNode(true);
      script2.removeAttribute(attr); 
      script.parentNode.insertBefore(script2, script);
      script.parentNode.removeChild(script);
    };
  }
  
  var asyncSupport = 'async' in document.createElement('script');
  
  // support async and defer
  opera.addEventListener('BeforeExternalScript', function(e) {
    var script = e.element;
    
    if (!asyncSupport && script.hasAttribute('async')) {
      setTimeout(load_script(script, 'async'), 10);
      return e.preventDefault();
    }
    
    if (script.hasAttribute('defer') && document.readyState != 'complete') {
      window.addEventListener('load', load_script(script, 'defer'), false);
      return e.preventDefault();
    }
  }, false);
})(this, this.document, this.opera);