You can save the code below and try it out.
In firefox,it's full browser grey,but in IE(IE7 to be exact),it's not working.
<!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>
<style text="text/css">
.overlay {
position:absolute;
top:0;
left:0;
width:100%;
height:100%;
z-index:1000;
background-color:grey;
}
</style>
</head>
<body style="font-size:62.5%;">
<div class="overlay"></div>
</body>
</html>