Nested grid tables.
```
% pandoc -f html -t markdown --columns=72
<table>
  <tr>
    <td>
      <table>
        <tr>
          <td>
            <table>
              <tr>
                <td>some text</td>
              </tr>
            </table>
          </td>
        </tr>
      </table>
    </td>
  </tr>
</table>
^D
+---------------------------------------------------------------------------+
| +-----------------------------------------------------------------------+ |
| |   -----------                                                         | |
| |   some text                                                           | |
| |   -----------                                                         | |
| +-----------------------------------------------------------------------+ |
+---------------------------------------------------------------------------+
```