Just use <th>
as the first element in the row. Then add the scope
attribute, which has no visual impact, but you could use it e.g. in CSS.
<table>
<tbody>
<tr>
<th scope="row">A</th>
<td>b</td>
</tr>
<tr>
<th scope="row">C</th>
<td>d</td>
</tr>
</tbody>
</table>