如何让弹出框永远在屏幕中间显示

.baocuncg{    background: rgba(0,0,0,0.7);    color: #ffffff;    border-radius: 5px;    text-align: center;    line-height: 40px;    z-index:99;    width: 300px;    height: 120px;    left:50%;/*FF IE7*/    top: 50%;/*FF IE7*/    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*/}