<style> in Gmail

Before

<style>
  .header { background-color: #6d28d9; padding: 32px; }
  .title  { color: #fff; font-size: 24px; }
</style>
<div class="header">
  <h1 class="title">Hello</h1>
</div>

After

<div style="background-color: #6d28d9; padding: 32px;">
  <h1 style="color: #fff; font-size: 24px;
             font-family: Arial, sans-serif; margin: 0;">
    Hello
  </h1>
</div>