.mydiv
{
    text-align: center;
    line-height: 40px;
    font-size: 12px;
    font-weight: bold;
    z-index: 999; /*优先级*/
    width: 300px;
    height: 120px;
    left: 50%;
    top: 50%;
    margin-left: -150px !important; /*FF IE7 该值为本身宽的一半 ，!important的出现就是为了让用户自己设置被执行语句的优先级*/
    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: #666;
    display: none;
    width: 100%;
    height: 100%;
    left: 0;
    top: 0; /*FF IE7*/
    filter: alpha(opacity=50); /*IE ，透明度*/
    opacity: 0.5; /*FF*/
    z-index: 1; /*优先级*/
    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);
}