绝对居中半透明弹出层

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>5DivCSS实现始终居中的半透明弹出层</title>
<style type="text/css">
<!--
html,body {height:100%; margin:0px; font-size:12px;}
.mydiv {
background-color: #FFCC66;
border: 1px solid #f00;
text-align: center;
line-height: 40px;
font-size: 12px;
font-weight: bold;
z-index:999;
width: 300px;
height: 120px;
left:50%;
top:50%;
margin-left:-150px!important;/*FF IE7 该值为本身宽的一半 */
margin-top:-60px!important;/*FF IE7 该值为本身高的一半*/
margin-top:0px;
position:fixed!important;/* FF IE7*/
position:absolute;/*IE6*/
_top:       expression(eval(document.compatMode &&
            document.compatMode=='CSS1Compat') ?
            documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2 :/*IE6*/
            document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2);/*IE5 IE5.5*/
}
.bg {
background-color: #666; display:none;
width: 100%;
height: 100%;
left:0;
top:0;/*FF IE7*/
filter:alpha(opacity=50);/*IE*/
opacity:0.5;/*FF*/
z-index:1;
position:fixed!important;/*FF IE7*/
position:absolute;/*IE6*/
_height:100px;
}

-->
</style>
<script language="javascript" type="text/javascript">
function showDiv(){
document.getElementById('popDiv').style.display='block';
document.getElementById('popIframe').style.display='block';
document.getElementById('bg').style.display='block';
}
function closeDiv(){
document.getElementById('popDiv').style.display='none';
document.getElementById('bg').style.display='none';
document.getElementById('popIframe').style.display='none';
}
</script>
</head>
<body>
<a href="javascript:showDiv()">点击这里弹出层</a>
<div id="popDiv" class="mydiv" style="display:none;">QQ:524994475<a href="javascript:closeDiv()">关闭窗口</a></div>
<div id="bg" class="bg" style="display:none;"></div>
</body>
</html>