(function(d) {
var iframe = d.body.appendChild(d.createElement('iframe')),
doc = iframe.contentWindow.document;
// style the iframe with some CSS
iframe.style.cssText = "position:absolute;200px;height:100px;left:0px;";
doc.open().write('<body onload="' + 'var d = document;d.getElementsByTagName(\'head\')[0].' + 'appendChild(d.createElement(\'script\')).src' + '=\'\/path\/to\/file\'">');
doc.close(); //iframe onload event happens
})(document);
http://www.cnblogs.com/beiyuu/archive/2011/07/18/iframe-tech-performance.html