用CSS实现文字垂直居中的代码

<style type="text/css">
<!--
.con_div{
width:400px;
height:300px;
border:1px solid #777;
text-align:center;
display:table-cell;
vertical-align:middle;
background:red;
color:#fff
}
/*FOR IE*/
.knowsky{
width:0;
height:100%;
display:inline-block;
vertical-align:middle;
}
-->
</style>

<div class="con_div">
<span class="knowsky"></span>
测试内容

</div>