For me none of the above worked. I think it is because I have two levels of header and a fixed width on level 1. So I couldn't align the text inside the corresponding columns on level 2.
+---------------------------+
| lvl 1 |
+---------------------------+
| lvl 2 col a | lvl 2 col b |
+---------------------------+
I had to use the combination of width:auto and text:align-center :
<th style="width:auto;text-align:center">lvl 2 col a</th>
<th style="width:auto;text-align:center">lvl 2 col b</th>