<style type="text/css">
	.black_overlay{
		position: absolute;
		top: 0%;
		left: 0%;
		width: 100%;
		height: 100%;
		background-color: black;
		z-index:1000;//使其div在高层，用户无法点击底层事件.
    	        opacity:.2;//支持firefox
    	        filter: alpha(opacity=20);//支持ie
       }
</style>
<div id="fade" class="black_overlay"></div>