table {
  width: 100%;
  border: 1px solid #000;
}
th.from, th.date {
  width: 15%
}
th.subject {
  width: 70%; /* Not necessary, since only 70% width remains */
}

<table>
  <thead>
    <tr>
      <th class="from">From</th>
      <th class="subject">Subject</th>
      <th class="date">Date</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>[from]</td>
      <td>[subject]</td>
      <td>[date]</td>
    </tr>
  </tbody>
</table>

The best practice is to keep your HTML and CSS separate for less code duplication, and for separation of concerns (HTML for structure and semantics, and CSS for presentation).

Note that, for this to work in older versions of Internet Explorer, you may have to give your table a specific width (e.g., 900px). That browser has some problems rendering an element with percentage dimensions if its wrapper doesn't have exact dimensions.