box-sizing in Yahoo Mail

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>