html+css+js
实现代码

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>DIV CSS遮罩层</title>
<script language="javascript" type="text/javascript">
function showdiv() {
            document.getElementById("bg").style.display ="block";
            document.getElementById("show").style.display ="block";
        }
function hidediv() {
            document.getElementById("bg").style.display ='none';
            document.getElementById("show").style.display ='none';
        }
</script>
<style type="text/css">
        #bg{ display: none;  position: absolute;  top: 0%;  left: 0%;  width: 100%;  height: 100%;  background-color: black;  z-index:1001;  -moz-opacity: 0.7;  opacity:.70;  filter: alpha(opacity=70);}
        #show{display: none;  position: absolute;  top: 25%;  left: 22%;  width: 53%;  height: 49%;  padding: 8px;  border: 8px solid #E8E9F7;  background-color: white;  z-index:1002;  overflow: auto;}
</style>
</head>
<body>
<input id="btnshow" type="button" value="Show" onclick="showdiv();"/>
<div id="bg" onclick="hidediv();"></div>
<div id="show">测试
<input id="btnclose" type="button" value="Close" onclick="hidediv();"/>
</div>
</body>
</html>