让一层div遮住全窗口 另一层显示登陆框自动居中的css代码

#div_mask        {            margin: 0 auto;            width: 100%;            height: 100%;            z-index: 200;            background-color: Black;            position: fixed;            left: 0;            top: 0;            opacity: 0.95;            moz-opacity:0.95;            filter:alpha(opacity=95);            display: none;        }        #div_login        {            position:absolute;            top:50%;            left:50%;            width: 300px;            height: 230px;            margin: -150px 0 0 -115px;            background-color: #CA8EFF;            vertical-align: middle;            display: none;                                }