如何让一个DIV水平，垂直方向都居中于浏览器？

<!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">
#mydiv{
    position:absolute;
    top:50%;
    left:50%;
    margin-left:-200px;
    margin-top:-150px;
    width:400px;
    height:300px;
    border:1px solid red;
    }
</style>
</head>

<body>
    <div id="mydiv"></div>
</body>
</html>

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