css div垂直居中

<!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>无标题文档</title>
<style type="text/css">
.main{
    position:absolute;
    top:50%;
    left:50%;
    width:200px;
    height:100px;
    margin-top:-50px;
    margin-left:-100px;

    border:1px solid red;
    line-height:30px;
    font-size:16px;
    text-align:center;
    }
</style>
</head>

<body>
<div class="main">王锋老师博客:http://blog.sina.com/wangfengteacher</div>
</body>
</html>
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