javascript弹出一个层并增加一个覆盖层

<!--内容层-->
<div id="ShopConfirmLayer" style="position:absolute;top:300px;left:100px;z-index:900;border:1px red solid;width:500px;display:none;">
<input name="button1" value="关闭" type="button" onclick="CloseShopConfirm()"/>
</div>
<!--覆盖层-->
<div id="webBgLayer" style="position:absolute;top:0px;left:0px;z-index:899;background-color:#ccc;height:100%;width:100%;display:none;-moz-opacity:0.5;filter:alpha(opacity=50);"></div>
<script type="text/javascript">
function ShopConfirm(str){
var ShopConfirmLayer=document.getElementById("ShopConfirmLayer");
var webBgLayer=document.getElementById("webBgLayer");
ShopConfirmLayer.innerHTML=str;
ShopConfirmLayer.style.display="";
ShopConfirmLayer.style.left=parseInt((document.documentElement.scrollWidth-ShopConfirmLayer.offsetWidth)/2)+document.documentElement.scrollLeft+"px";
ShopConfirmLayer.style.top=Math.abs(parseInt((document.documentElement.clientHeight-ShopConfirmLayer.offsetHeight)/2))+document.documentElement.scrollTop+"px";
webBgLayer.style.display="";
webBgLayer.style.height=document.documentElement.scrollHeight+"px";
}
function CloseShopConfirm(){
var ShopConfirmLayer=document.getElementById("ShopConfirmLayer");
var webBgLayer=document.getElementById("webBgLayer");
ShopConfirmLayer.style.display="none";
webBgLayer.style.display="none";
}
</script>注意上面的代码头部DTD应该是这样：