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;

    }