Question : Graying out the background

When a pop up opens how do you gray out the page behind it?

Answer : Graying out the background

Working sample using javascript and css attached.
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<!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>Untitled</title>
<style type="text/css">
  <!--
    .screen
    {
        position:fixed;
        top:0px;
        left:0px;
        display:none;
	width:100%;
	height:100%;
        background-color: #333; 
        -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)"; 
        filter: alpha(opacity=50); 
        opacity: 0.5;
        z-index:200;
        visibility:hidden;
    }
    .popup {
        position: absolute;
        top: 100px;
        left: 425px;
        width: 120px;
        height: 120px; 
        background-color:#fff;
        z-index:300;
        visibility:hidden;
    }
  -->
</style>
</head>
<body>
<br />
<p>
<a href="javascript:show();">Show</a>
</p>
<br />
<div id="screen" class="screen"></div>
<div id="popup" class="popup"><a href="javascript:hide();">Hide</a></div>
<script language="javascript" type="text/javascript">
function show() {
    document.getElementById("screen").style.display = 'block';
    document.getElementById("screen").style.visibility = 'visible';
    document.getElementById("popup").style.visibility = 'visible';
}
function hide() {
    document.getElementById("screen").style.display = 'none';
    document.getElementById("screen").style.visibility = 'hidden';
    document.getElementById("popup").style.visibility = 'hidden';
}
</script>
</body>
</html>