<style>
.mask {
position: absolute; top: 0px; filter: alpha(opacity=60); background-color: #777;
z-index: 1002; left: 0px;
opacity:0.5; -moz-opacity:0.5;
}
</style>
<body>
<div id="mask" class="mask"></div>
</body>
<script>
$("#mask").css("height",$(document).height());
$("#mask").css("width",$(document).width());
$("#mask").show();
</script>