简单的遮罩层实现

<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: 70%;
height: 50%;
padding: 8px;
border: 8px solid #E8E9F7;
background-color: white;
z-index: 1002;
overflow: auto;
}
</style>

<div id="bg" runat="server" style="display: none;">
</div>
<div id="show" runat="server" style="display: none;">

</div>