弹出层始终出现在屏幕中间

http://www.68ecshop.com/article-837.html


<!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>
.joinclub {
	width: 440px;
	height: 282px;
	border: 1px solid #6E665A;
	border-radius: 5px;
	background: #fff;
	padding-top: 10px;
	position: absolute;
	z-index: 99999999;
	display: none;
	left: 50%;/*FF IE7*/
	top: 50%;/*FF IE7*/
	margin-left: -220px!important;/*FF IE7 该值为本身宽的一半 */
	margin-top: -140px!important;/*FF IE7 该值为本身高的一半*/
	margin-top: 0px;
	position: fixed!important;/*FF IE7*/
	position: absolute;/*IE6*/
 _top:       expression(eval(document.compatMode && document.compatMode=='CSS1Compat') ? documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2 :/*IE6*/
 document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2);/*IE5 IE5.5*/
}
.bg123 {
	background-color: #ccc;
	width: 100%;
	height: 100%;
	z-index: 99999998;
	left: 0;
	top: 0;/*FF IE7*/
	filter: alpha(opacity=50);/*IE*/
	opacity: 0.5;/*FF*/
	position: fixed!important;/*FF IE7*/
	position: absolute;/*IE6*/
 _top:       expression(eval(document.compatMode && document.compatMode=='CSS1Compat') ? documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2 :/*IE6*/
 document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2);/*IE5 IE5.5*/
}
</style>
</head>
 
<body>
<div style="width:100%; height:500px; border:1px solid red"></div>
<a href="#" onclick="join_club()">点击我</a>
</body>
<div class="joinclub" id="joinclub" style="display:none">我是弹出菜单<input type="button" value="关闭我" onclick="closeCustomer()" /></div>
<div id="bg123" class="bg123" style="display:none;"></div>
<script type="text/javascript">
function closeCustomer(){
	document.getElementById("joinclub").style.display='none';
	document.getElementById("bg123").style.display='none';
	}
function join_club()
{
	document.getElementById('joinclub').style.display = 'block';
	document.getElementById('bg123').style.display = 'block';
}
</script>
</html>


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