table 的rolspan和rowspan

如图所示啦，容易让初学者混乱的两个东西
仔细看看分析下呢，就比较简单了

<table width="300" border="2">
  <tr>
    <th scope="col">&nbsp;</th>
    <th scope="col">&nbsp;</th>
    <th scope="col">&nbsp;</th>
  </tr>
  <tr>
    <td rowspan="2">1</td>
    <td>2a</td>
    <td>3a</td>
  </tr>
  <tr>
    <td>2</td>
    <td>3</td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td colspan="2">4</td>
  </tr>
</table>