http://www.yicha.net/yicha/code2/code.asp?id=1192
<!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=gb2312" /><title>始终居中的弹出层</title><style type="text/css"><!--html,body {height:100%; margin:0px; font-size:12px;}.mydiv {background-color: #FFCC66;border: 1px solid #f00;text-align: center;line-height: 40px;font-size: 12px;font-weight: bold;z-index:999;width: 300px;height: 120px;left:50%;top:50%;margin-left:-150px!important;/*FF IE7 该值为本身宽的一半 */margin-top:-60px!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*/
}
.bg,.popIframe {background-color: #666; display:none;width: 100%;height: 100%;left:0;top:0;/*FF IE7*/filter:alpha(opacity=50);/*IE*/opacity:0.5;/*FF*/z-index:1;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);}.popIframe {filter:alpha(opacity=0);/*IE*/opacity:0;/*FF*/}--></style><script language="javascript" type="text/javascript">function showDiv(){document.getElementById('popDiv').style.display='block';document.getElementById('popIframe').style.display='block';document.getElementById('bg').style.display='block';}function closeDiv(){document.getElementById('popDiv').style.display='none';document.getElementById('bg').style.display='none';document.getElementById('popIframe').style.display='none';
}</script></head><body><div id="popDiv" class="mydiv" style="display:none;">网页设计大本营欢迎你！<br/>Q群号:29032448<br/><a href="javascript:closeDiv()">关闭窗口</a></div><div id="bg" class="bg" style="display:none;"></div><a href="javascript:showDiv()">点击这里弹出层</a><iframe id='popIframe' class='popIframe' frameborder='0' ></iframe></body></html>
