是层垂直水平居中

    <style type="text/css">      
    <!--       
    div {      
            position:absolute;      
            top:50%;      
            left:50%;      
            margin:-100px 0 0 -100px;      
            width:200px;      
            height:200px;      
            border:1px solid red;      
        }      
    -->      
    </style>