css遮盖层DEMO

.css

       .pay_div {
            width: 100%;
            position: absolute;
            z-index: 999;
            background-color: #FFFFFF;
            list-style-type: none;
            bottom: 0px;
            display: none;
        }
        .zhegai {
            width: 100%;
            height: 100%;
            background-color: #000;
            filter: alpha(opacity=50);
            -moz-opacity: 0.5;
            opacity: 0.5;
            position: absolute;
            left: 0px;
            top: 0px;
            display: none;
            z-index: 100;
        }

html

 <div id="Cover_layer" class="zhegai">  
 </div>

 <div id="pay_div" class="pay_div"> 
 </div>