背景遮罩层铺满屏

//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"

});