CSS创建一个遮罩层

   .layer{ 
    width: 100%;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    -moz-opacity: 0;
    filter: alpha(opacity=50);
    z-index: 99;
    height: 100%;
    margin:0 auto;
    }