表格感应鼠标背景变色的问题

Question

<style> 
TD 
{ 
margin-left:6px; font-size: 10pt; font-family: 宋体; text-valign: middle 
} 
.button1 
{ 
background-color: #F4F4FF; 
border-left: 1px solid #COCOCO; 
border-right: 1px solid #COCOCO; 
border-top: 1px solid #COCOCO; 
border-bottom: 1px solid #COCOCO; 
} 
.button0 
{ 
background-color: #FFFFFF; 
border-left: 0px solid #FFFFFF; 
border-right: 0px solid #FFFFFF;
border-top: 0px solid #FFFFFF;
border-bottom: 0px solid #FFFFFF;} 
</style> 
<table border="0" width="100%" cellspacing="0" cellpadding="0" id="table1" height="71">
<tr>
<td value='button' onmouseover="this.className='button1'" onmouseout="this.className='button2'"><span id="m1">鼠标经过该行文字背景不变色</span></td>
</tr>
<tr>
<td value='button' onmouseover="this.className='button1'" onmouseout="this.className='button2'">鼠标经过该行文字背景变色</td>
</tr>
</table>