<!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>定义自适应界面的居中</title>
<style>
.aa{ position:absolute;
height: 200px;
width: 200px;
margin-top:-100px;
margin-left:-100px;
top: 50%;
left:50%;
z-index:1; background-color:#ccc;}
</style>
</head>
<body>
<div class="aa"></div>
</body>
</html>
原文：http://www.cnblogs.com/Better-Me/p/4087116.html