See W3C documentation for the XHTML table model.
Container element for the table header rows of a table
(tr)+
<tr> Table Row (XHTML table model), one or more
...
<floats-group>
<table-wrap>
...
<graphic copyright="inherit" id="tb3a"
xlink:href="jpa_14_2_160_tb3a.tif" />
<table>
<thead align="left">
<tr>
<th colspan="1" rowspan="1"></th>
<th colspan="5" align="center" rowspan="1">Variables</th>
</tr>
...
</thead>
<tbody>
<tr>
<td colspan="1" rowspan="1">p<sub location="post" arrange="stack">1</sub></td>
<td colspan="1" rowspan="1">a<sub location="post" arrange="stack">11</sub></td>
<td colspan="1" rowspan="1">a<sub location="post" arrange="stack">12</sub></td>
<td colspan="1" rowspan="1">...</td>
<td colspan="1" rowspan="1">...</td>
<td colspan="1" rowspan="1">a<sub location="post" arrange="stack">1n</sub></td>
</tr>
...
</tbody>
</table>
</table-wrap>
</floats-group>
...
xhtml-table-1.mod