Вместо повторения tbody
много раз вы можете использовать следующую структуру:
table {
width: 100%;
border-collapse: collapse;
table-layout: fixed;
border: none;
}
tbody table {
box-shadow: 0 0 0 1px black;
border-radius: 8px;
}
<table>
<thead>
<tr>
<th>Firstname</th>
<th>Lastname</th>
</tr>
</thead>
<tbody>
<tr>
<td colspan="2">
<table>
<tr>
<td>Jill</td>
<td>Smith</td>
</tr>
<tr>
<td>Eve</td>
<td>Jackson</td>
</tr>
</table>
</td>
</tr>
<tr><td colspan="2" height="20"></td></tr>
<tr>
<td colspan="2">
<table>
<tr>
<td>Jill</td>
<td>Smith</td>
</tr>
<tr>
<td>Eve</td>
<td>Jackson</td>
</tr>
</table>
</td>
</tr>
<tr><td colspan="2" height="20"></td></tr>
<tr>
<td colspan="2">
<table>
<tr>
<td>Jill</td>
<td>Smith</td>
</tr>
<tr>
<td>Eve</td>
<td>Jackson</td>
</tr>
</table>
</td>
</tr>
<tr><td colspan="2" height="20"></td></tr>
</tbody>
</table>