DHTML动态创建一个弹出遮罩层

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
  <script language="javascript">
   function showdialog()
   {
    var divmask=document.createElement("div");
    divmask.style.cssText="position:absolute;left:0px;top:0px;100%;height:100%;z-index:4;background-color:yellow;filter:Alpha(opacity=80)";
    document.body.appendChild(divmask);
   }
  </script>
</head>
<body topmargin="0" leftmargin="0">
<center>
    <button id="test" onclick="javascript:showdialog();">
    <br/>
    弹出层
    <br/>
    </div>

</center>
</body>
</html>