loading完成， 加载提示条自动消失
<!doctype html>
<html>
<head>
<script>
window.onload=function(){
var a = document.getElementById("loading");
a.parentNode.removeChild(a);
}
document.write('<div id="loading" style="background:green;color:#FFF;width:80px;padding:5px;">正在读取...</div>');
</script>
</head>
<body>
<div style="height:500px">
<iframe src="http://www.baidu.com" width="100%" height="100%" frameborder="0" marginwidth="0" marginheight="0"></iframe>
</div>
</body>
</html>
