Gmail converts this:
<td style=“height: 20px;”></td>
<td style=“min-height: 20px;”></td>
The min-height property does not have the same effect as the “height” property in TDs. This seems to have a universal effect in all browsers. By adding a space or a <br /> inside the empty cell, it resolves the problem.
<td style=“min-height: 20px;”> </td>