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.