See the width section of Help:Table. This example table needs to be on this separate page in order to test it more easily in narrower browser windows. Especially in mobile view on cell phones in portrait orientation.
You can see the problem without having to go to a cell phone. Click on the mobile view link at the bottom of this Wikipedia page. Then narrow your browser window. Note that the center column does not narrow in any circumstance. This is why the use of style=width:Xem should be avoided in tables.
Center header width setting:
style=width:20em | Center
| Left | Center | Right |
|---|---|---|
| Top left cell | Top center cell | Top right cell |
| Middle left cell | Middle center cell | Middle right cell |
| Bottom left cell | Bottom center cell | Bottom right cell |