You can use the Colspan and rowspan attributes to set how far each cell goes across rows and columns.
For example:
<table>
<tbody>
<tr>
<td rowspan="2">Top Left Header</td>
<td colspan="3">Call Standard</td>
</tr>
<tr>
<td>Flagged</td>
<td>Percent</td>
<td>Days</td>
</tr>
</tbody>
</table>
Note that the table ends up with 4 columns. The first row defines one column which crosses 2 rows, and a column which crosses 3 columns.
The second row just fills in the "missing" columns; ignoring the first one because it was defined previously.