@media in Fastmail

Before

<table width="800">
  <tr>
    <td width="400">Left Column</td>
    <td width="400">Right Column</td>
  </tr>
</table>
<style>
  @media (max-width: 600px) {
    table { width: 100% !important; }
    td { display: block !important; width: 100% !important; }
  }
</style>

After

<!-- Single-column layout that works without @media -->
<table role="presentation" width="100%"
  style="max-width: 600px;" cellpadding="0"
  cellspacing="0" border="0">
  <tr>
    <td style="padding: 16px;">Left Column</td>
  </tr>
  <tr>
    <td style="padding: 16px;">Right Column</td>
  </tr>
</table>