Use W3C documentation for the XHTML1.1 table model. (http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_tablemodule)
One cell in the table header, as opposed to an ordinary cell in a table
Any combination of:
In the following example, the table is wrapped in an <alternatives> element together with a graphic showing the formatted table:
...
<table-wrap>
<alternatives>
<table rules="rows">
<thead>
<tr>
<th>Color</th>
<th>Size</th>
<th>Price</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="3">Green</td>
<td>small</td>
<td>$3.25</td>
</tr>
<tr>
<td>medium</td>
<td>$2.25</td>
</tr>
<tr>
<td>large</td>
<td>$1.15</td>
</tr>
<tr>
<td rowspan="3">Red</td>
<td>small</td>
<td>$3.25</td>
</tr>
<tr>
<td>medium</td>
<td>$5.25</td>
</tr>
<tr>
<td>large</td>
<td>$9.95</td>
</tr>
</tbody>
</table>
<graphic xlink:href="images/tablesample1">
<alt-text>A table with columns for color, size, and price.</alt-text>
<long-desc>The table has three columns, labeled "Color",
"Size", and "Price".
It has six rows; reading across, they are:
Green, small, $3.25,
Green, medium, $2.25,
Green, large, $1.15;
Red, small, $3.25,
Red, medium, $5.25,
Red, large, $9.95.
(Tbe words "Green" and "Red" occur only once each, in an area
three rows in height.)
</long-desc>
</graphic>
</alternatives>
</table-wrap>
...
The graphic rendering of the table is:

xhtml-table-1.mod