Display cell below previous cell

I have a structure per the following:

<table>
    <tr>
        <td>a</td>
        <td>b</td>
        <td>c</td>
    </tr>
    <tr>
        <td>x</td>
        <td>y</td>
        <td>z</td>
    </tr>
</table>

And, using CSS, I want to see this following result (without visible borders):

+---+---+
| a | b |
|   | c |
+---+---+
| x | y |
|   | z |
+---+---+

To elaborate on my motivation: The table is an overview of job history, and the columns are period, position, and employer. I want to put the employer below the position for readability.