HTML

The following HTML5 elements are not supported by all email clients and should not be used:

<article>
<aside>
<details>
<figcaption>
<figure>
<footer>
<header>
<main>
<mark>
<nav>
<section>
<summary>
<time>

Other standard HTML elements like <div>, <span>, heading elements (<h1>, <h2>, etc.), paragraphs (<p>), lists (<ul>, <ol>, <li>) are supported everywhere.