The following three examples are 2 nested tables.
The inner table has a height of 100%, the outer table has a height set of 200px, on the <table>, <tr> and <td> respectively.
In all examples the inner table should completely fill the outer table. (with respect of the padding and border ofcourse)

outer table has height:200px on <table>

outer table has height:200px on <tr>

outer table has height:200px on <td>

outer table has no height set, inner table has a height of 50%. Here the height should be set to auto, because the height of the outer td depends on the content.