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;
     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*/
}