<!--
.mydiv {
background-color: #FFCC66;
border: 1px solid #f00;
text-align: center;
line-height: 50px;
font-size: 12px;
font-weight: bold;
width: 300px;
height: 100px;
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:-50px!important;/*FF IE7 该值为本身高的一半*/
margin-left:0px;
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*/
_left:      expression(eval(document.compatMode &&
document.compatMode=='CSS1Compat') ?
documentElement.scrollLeft + (document.documentElement.clientWidth-this.offsetWidth)/2 :/*IE6*/
document.body.scrollLeft + (document.body.clientWidth - this.clientWidth)/2);/*IE5 IE5.5*/

}