Google is bringing AMP, its cut-down version of HTML, to email. Starting today, Gmail on the Web will be able to support embedded AMP content, with support rolling out to mobile clients later ...

A Security Problem HTML-based email can be susceptible to viruses, because HTML supports JavaScript, and JavaScript is a language that activates functions.

Always test your pages using Google’s own tools and third-party resources to view the rendered HTML to make sure all your content is showing up.