css 实现浏览器窗口居中div层方法

<!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=utf-8" />
<title>层居中</title>
<style>
#overlay {
 position:absolute;
 top:0;
 left:0;
 width:100%;
 height:100%;
 background:#000;
 opacity:0.5;
 filter:alpha(opacity=50);
}
#win {
 position:absolute;
 top:50%;
 left:50%;
 width:400px;
 height:200px;
 background:#fff;
 margin:-102px 0 0 -202px;
 line-height: 200px;
 text-align: center;
 border: 4px solid #CCC;
}
</style>
<script>
</script>
</head>
<body>
<div id="overlay"></div>
<div id="win">层居中：）</div>
</body>
</html>