请问:如何把一个层显示在屏幕的正中?

<BODY onload="setPos()" onresize="setPos()">
<div style="background-color:yellow;width:0;height:0;position:absolute" id=div1>
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<SCRIPT LANGUAGE="JavaScript">
<!--
function setPos(){
div1.style.left=(document.body.offsetWidth-div1.offsetWidth)/2
div1.style.top=(document.body.offsetHeight-div1.offsetHeight)/2
}//-->
</SCRIPT>
</div>
</BODY>