How to add a table with 100% width with equal size columns with CSS?

To add a table with 100% width with equal size columns with CSS, we set the width of each column.

For instance, we write

<table width="400px">
  <tr>
    <td width="100px"></td>
    <td width="100px"></td>
    <td width="100px"></td>
    <td width="100px"></td>
  </tr>
</table>

to set the width of each td element to make the column width 100px.