设置DIV显示在屏幕中间

/*设置客户端的高和宽*/
function getClientBounds(){
var clientWidth;
var clientHeight;

clientWidth = document.compatMode == "CSS1Compat" ?
document.documentElement.clientWidth : document.body.clientWidth;
clientHeight = document.compatMode == "CSS1Compat" ?
document.documentElement.clientHeight : document.body.clientHeight;

return {width: clientWidth, height: clientHeight};
}

/*设置客户端的高和宽*/
function divcenter(){
var divId=document.getElementById('mxh');
var rr=new getClientBounds();
divId.style.display='block';
divId.style.left=(rr.width-divId.clientWidth)/2+document.body.scrollLeft;
divId.style.top=(rr.height-divId.clientHeight)/2+document.body.scrollTop;
}