层罩的CSS设置

自己改写了网上拷贝的层罩代码，用后发现当有滚动条的时候，层罩能盖住整个页面，而当页面的内容只有几行的时候（如页面高的1/3）层罩只能盖住有内容的部分，不能盖住整个页面（层罩背景色不一样，很难看），该怎样处理？这些代码是不是所有的浏览器都有效？。
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.divCover
{
filter: alpha(opacity=30);
-moz-opacity: 0.3;
opacity: 0.3;
background-color: #000;
width: 100%;
height: 100%;
z-index: 1000;
position: absolute;
left: 0;
top: 0;
overflow: hidden;
}

function objDisplay(objId,value){
document.getElementById(objId).style.display = value ;
}
  function $(id) {
            return (document.getElementById) ? document.getElementById(id) : document.all[id];
        }

        function showDivCover(divCover) {
            var range = getRange();
            $(divCover).style.width = range.width + "px";
            $(divCover).style.height = range.height + "px";
            $(divCover).style.display = "block";
            }
        function getRange() {
            var top = document.body.scrollTop;
            var left = document.body.scrollLeft;
            var height = document.body.clientHeight;
            var width = document.body.clientWidth;

            if (top == 0 && left == 0 && height == 0 && width == 0) {
                top = document.documentElement.scrollTop;
                left = document.documentElement.scrollLeft;
                height = document.documentElement.clientHeight;
                width = document.documentElement.clientWidth;
            }
            return { top: top, left: left, height: height, width: width };
        }


  <div id="divCover" style="display: none;" class="divCover" /> --------------------编程问答--------------------
  top = document.documentElement.scrollTop;
  left = document.documentElement.scrollLeft;
  var Width =  document.body.offsetWidth+'px';

  var sClientHeight =  document.body.offsetHeight;
  var sScrollTop = document.body.scrollTop+document.documentElement.scrollTop;
  height=(sClientHeight + sScrollTop) + "px";

return{ top: top, left: left, height: height, width: width };

补充：.NET技术 ,  ASP.NET