背景遮罩层展满屏

//背景

#bg{ display: none; ?position: absolute; ?top: 0%; ?left: 0%; ?width: 100px; ?height: 100px; ?background-color: black; ?z-index:1001; ?-moz-opacity: 0.7; ?opacity:.70; ?filter: alpha(opacity=70);}

var mask = document.getElementById("bg");

var b = document.documentElement?document.documentElement:document.body;

mask.style.width = b.clientWidth+"px";

mask.style.height = b.scrollHeight+"px";

$("."+clazz).css({

left:($("body").width()-$("."+clazz).width())/2-20+"px",

top:($(window).height()-$("."+clazz).height())/2+$(window).scrollTop()+"px",

display:"block"

});