Graying out a page background for overlay or mouse over page

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<script type="text/javascript">
function showPopUp(el) {
var cvr = document.getElementById("cover")
var dlg = document.getElementById(el)
cvr.style.display = "block"
dlg.style.display = "block"
if (document.body.style.overflow = "hidden") {
cvr.style.width = "1024"
cvr.style.height = "100%"
}
}

function closePopUp(el) {
var cvr = document.getElementById("cover")
var dlg = document.getElementById(el)
cvr.style.display = "none"
dlg.style.display = "none"
document.body.style.overflowY = "scroll"
}

</script>
<style type="text/css">
#cover {
display:none;
position:absolute;
left:0px;
top:0px;
width:100%;
height:100%;
background:gray;
filter:alpha(Opacity=50);
opacity:0.5;
-moz-opacity:0.5;
-khtml-opacity:0.5
}

#dialog {
display:none;
left:200px;
top:200px;
width:300px;
height:300px;
position:absolute;
z-index:100;
background:white;
padding:2px;
font:10pt tahoma;
border:1px solid gray
}

</style>
</head>
<body>
<div id="cover"></div>
<div id="dialog">
My Dialog Content
<br><input type="text">
<br><input type="button" value="Submit">
<br><a href="#" onclick="closePopUp('dialog');">[Close]</a>
</div>
<a href="#" onclick="showPopUp('dialog');">Show</a>
</body>
</html>

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