JS implementation mask layer effect of a simple example

function show(){ 
    var cover = document.getElementById("cover"); 
    cover.style.width = document.documentElement.scrollWidth+"px"; 
    cover.style.height = document.documentElement.scrollHeight+"px"; 
    cover.style.display = "block"; 
} 
#cover{ 
    background:gray; 
    position:absolute; 
    left:0px; 
    top:0px; 
    display:none; 
    z-index:20; 
    filter:alpha(opacity=60); 
    opacity:0.6 !important; 
    }