<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>提示信息</title>
<script type="text/javascript">
window.onload = function() {
var div = document.getElementById("showDiv");
div.style.position= "absolute";
div.style.top = (document.documentElement.scrollTop + (document.documentElement.clientHeight -div.offsetHeight) / 2)
+ "px";
div.style.left = (document.documentElement.scrollLeft + (document.documentElement.clientWidth - div.offsetWidth) / 2)+ "px";
};
</script>
</head>
<body></body>
</html>
2、下面也可以实现，不过要去掉上面的" <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> "这行代码。
<script type="text/javascript">
window.onload = function() {
var div = document.getElementById("showDiv");
div.style.position= "absolute";
div.style.top = (document.body.scrollTop + (document.body.clientHeight -div.offsetHeight) / 2)
+ "px";
div.style.left = (document.body.scrollLeft + (document.body.clientWidth - div.offsetWidth) / 2)+ "px";
};
</script>
3、如果需要解决兼容性问题，可以使用jquery，如下：
window.onload = function() {
var div = document.getElementById("showDiv");
div.style.position= "absolute";
div.style.top = (($(document).height()-div.offsetHeight) / 2)+ "px";
div.style.left = (($(document).width() - div.offsetWidth) / 2)+ "px";
};