兼容主流浏览器的背景遮盖和居中浮动层的CSS样式

html, body{    height: 100%;    margin: 0px;    font-size: 12px;} .mydiv{    background: #f1f1f1;    padding: 15px 30px;    border: 5px solid #ccc;    z-index: 99;    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*/} .bg{    background-color: #ccc;    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); /*IE5 IE5.5*/}