怎样使一个层垂直居中于浏览器中
<!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>
<title> new document </title>
<meta name="generator" content="editplus" />
<meta name="author" content="" />
<meta name="keywords" content="" />
<meta name="description" content="" />
<style type="text/css">
<!--
div{
border:1px solid red;
height:200px;
left:50%;
margin:-100px 0 0 -100px;
position:absolute;
top:50%;
width:200px;
}
-->
</style>
</head>
<body>
<div></div>
</body>
</html>
