html rowspan的怪象，求大神答疑！！！

偶然发现当表格的首行都出现rowspan属性，值大于2时，会出现渲染异常问题。如下：

rowspan 都等于 2 时

<table>
<thead>
  <tr>
    <th>月份</th>
    <th>存款</th>
    <th rowspan="3">节假日的储蓄！</th>
  </tr>
</thead>
<tbody>
  <tr>
    <th rowspan="2">0</th>
    <th rowspan="2">0</th>
    <th rowspan="2">0！</th>
  </tr>
  <tr>
    <td>一月</td>
    <td>￥3000</td>
    <td rowspan="0">￥6000</td>
  </tr>
  <tr>
    <td>二月</td>
    <td>￥4000</td>
  </tr>
</tbody>
</table>

rowspan 都等于 3 时

<table>
<thead>
  <tr>
    <th>月份</th>
    <th>存款</th>
    <th rowspan="3">节假日的储蓄！</th>
  </tr>
</thead>
<tbody>
  <tr>
    <th rowspan="3">0</th>
    <th rowspan="3">0</th>
    <th rowspan="3">0！</th>
  </tr>
  <tr>
    <td>一月</td>
    <td>￥3000</td>
    <td rowspan="0">￥6000</td>
  </tr>
  <tr>
    <td>二月</td>
    <td>￥4000</td>
  </tr>
</tbody>
</table>