纯CSS实现图片水平垂直居中于DIV（图片未知宽高）

<div class="demo"><a href="#"><img src="http://nec.netease.com/img/l/1.jpg" alt=""/></a></div>

<style>

/*For Firefox Chrome*/
.demo{border:1px #ddd solid;width:210px;height:150px;overflow:hidden;text-align:center;display:table;margin:50px;}
.demo a{display:table-cell;vertical-align:middle;width:200px;height:140px;}
.demo a img{border:1px #ddd solid;margin:0 auto;max-width:200px;max-height:140px;}

/*For IE7*/
*+html .demo a{position:absolute;top:50%;width:100%;text-align:center;height:auto;}
*+html .demo a img{position:relative;top:-50%;left:-50%;}

/*For IE6*/
*html .demo a{position:absolute;top:51%;width:100%;text-align:center;height:auto;display:block;}
*html .demo a img{position:relative;top:-50%;left:-50%;width:expression(this.width>200?“200px”:“auto”);height:expression(this.height>140?“140px”:“auto”);}

</style>