Not all HTML tags are supported in Gmail actions. Below is a list of supported and unsupported tags. If an unsupported tag is added to the body of the email, it will not be sent with the message.
Supported HTML tags
<a>
<abbr>
<acronym>
<address>
<area>
<b>
<bdo>
<big>
<blockquote>
<br>
<button>
<caption>
<center>
<cite>
<code>
<col>
<colgroup>
<dd>
<del>
<dfn>
<dir>
<div>
<dl>
<dt>
<em>
<fieldset>
<font>
<form>
<h1>
<h2>
<h3>
<h4>
<h5>
<h6>
<hr>
<i>
<img>
<input>
<ins>
<kbd>
<label>
<legend>
<li>
<map>
<menu>
<ol>
<optgroup>
<option>
<p>
<pre>
<q>
<s>
<samp>
<select>
<small>
<span>
<strike>
<strong>
<sub>
<sup>
<table>
<tbody>
<td>
<textarea>
<tfoot>
<th>
<thead>
<u>
<tr>
<tt>
<u>
<ul>
<var>
Unsupported HTML tags
<applet>
<article>
<aside>
<audio>
<base>
<basefont>
<bdi>
<bgsound>
<blink>
<body>
<aside>
<audio>
<base>
<basefont>
<bdi>
<bgsound>
<blink>
<body>
<canvas>
<content>
<data>
<datalist>
<decorator>
<details>
<dialog>
<element>
<embed>
<figcaption>
<figure>
<footer>
<frame>
<frameset>
<head>
<header>
<hgroup>
<html>
<iframe>
<isindex>
<keygen>
<link>
<listing>
<main>
<mark>
<marquee>
<menuitem>
<meta>
<meter>
<nav>
<nobr>
<noframes>
<noscript>
<object>
<output>
<param>
<picture>
<progress>
<rp>
<rt>
<ruby>
<script>
<section>
<shadow>
<source>
<spacer>
<style>
<summary>
<template>
<time>
<title>
<track>
<video>
<wbr>
<xmp>