così:

codice:
<table border="1" width="100%" style="border: 1 dashed #00FF00">
  <tr>
    <td width="50%"></td>
    <td width="50%"></td>
  </tr>
  <tr>
    <td width="50%"></td>
    <td width="50%"></td>
  </tr>
</table>