DIV水平垂直居中 兼容多种浏览器
第一种：全CSS控制，层漂浮（适用于做登陆页面）
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<style type="text/css">
#divcenter{
position:absolute;/*层漂浮*/
top:50%;
left:50%;
width:300px;
height:300px;
margin-top:-150px;/*注意这里必须是DIV高度的一半*/
margin-left:-150px;/*这里是DIV宽度的一半*/
background:yellow;
border:1px solid red; }
</style>
</head>
<body>
<div id="divcenter"> this is a test </div>
</body>
</html>
第二种：JS + CSS控制，不漂浮（适用于做登陆页面）
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<script type="text/javascript">
function cen(){
var divname = document.all("testdiv");
//居中高度等于页面内容高度减去DIV的高度 除以2
var topvalue = (document.body.clientHeight - divname.clientHeight)/2;
divname.style.marginTop = topvalue;
}
//页面大小发生变化时触发
window.onresize = cen;
</script>
</head>
<body style="height:100%; width:100%; text-align:center;" onload=cen()>
<div id = "testdiv" name="testdiv" style="margin:0 auto; border:1px solid red; height:400px; width:400px;">
DIV居中演示
</div>
</body>
</html>
第三种：最简单适用的一种 上下左右都居中，兼容所有浏览器
<div style="position:absolute; top:50%; height:240px;border:1px solid red; margin:0 auto; margin-top:-120px; width:300px; left:50%; margin-left:-150px;"></div>