请教一个JS问题 请帮帮忙

谢谢楼上 虽然不太懂 也要谢谢你

<script type="text/javascript">
function changeCss(){
var t1 = document.getElementById("td1");
var t2 = document.getElementById("td2");
t1.className="tdclass_common";
t2.className="tdclass_common";
}
</script>
<ul class="subMenu" >
<table width="168" border="0">
<tr>
<td class="tdclass_act" id="td1"
onclick="javascript:changeCss();this.className='tdclass_act';"><li><a href="javascript:void(0)"><img src="123.png" width="168" height="57"></a></li></td>
</tr>
<tr>
<td class="tdclass_common" id="td2"
onclick="javascript:changeCss();this.className='tdclass_act';"><li><a href="javascript:void(0);"><img src="456.png" width="168" height="57"></a></li></td>
</tr>
</table>
</ul>
<div class="subContent">
<ul style="display:block; height:auto;">内容</ul>
<ul style=" height:auto;">内容</ul>
<div>
奉上代码