html表格单元格焦点事件问题

 <!DOCTYPE html>
<html>
  <head>
    <title>focus2.html</title>

    <meta name="keywords" content="keyword1,keyword2,keyword3">
    <meta name="description" content="this is my page">
    <meta name="content-type" content="text/html; charset=UTF-16">

    <!--<link rel="stylesheet" type="text/css" href="./styles.css">-->

  </head>

  <body>

    <table border="1">
    <tr>
    <td colspan="4" >学生信息</td>
    </tr>
    <tr>
    <th>姓名</th>
    <th>年龄</th>
    <th colspan="2">电话</th>
    </tr>

    <tr>
    <td onfocus="this.style.backgroundColor='#00f';"    onblur="this.style.backgroundColor='#fff';">刘勇</td>
    <td onfocus="this.style.backgroundColor='#00f';"    onblur="this.style.backgroundColor='#fff';">20</td>
    <td onfocus="this.style.backgroundColor='#00f';"    onblur="this.style.backgroundColor='#fff';">0732-88756773</td>
    </tr>
    </table>
  </body>
</html>

为什么获得焦点后颜色不会变呢 ，或者还是它根本没有获得焦点呀

td获取不到焦点的，给里面的内容添加一个a标签，a可以得到焦点


    <table border="1">
        <tr>
            <td colspan="4">学生信息</td>
        </tr>
        <tr>
            <th>姓名</th>
            <th>年龄</th>
            <th colspan="2">电话</th>
        </tr>

        <tr>
            <td><a href="" onfocus="this.parentNode.style.backgroundColor='#00f';" onblur="this.parentNode.style.backgroundColor = '#fff';">刘勇</a></td>
            <td><a href="" onfocus="this.parentNode.style.backgroundColor='#00f';" onblur="this.parentNode.style.backgroundColor = '#fff';">20</a></td>
            <td><a href="" onfocus="this.parentNode.style.backgroundColor='#00f';" onblur="this.parentNode.style.backgroundColor = '#fff';">0732-88756773</a></td>
        </tr>
    </table>