GMail (and Fastmail) are rendering the email. It just happens that the email and we webbrowser are both HTML. In no case should they just literally forward the email HTML to the browser. They scrub JavaScript, non-whitelisted HTML elements, rewrite links/external resources including tracking pixels.
You can see the raw email with "show original" in the options
chrisjj|7 months ago
Any documenation on that list?
meinersbur|7 months ago
I don't think there is an official list. It is security-related and changes over time
Redoubts|7 months ago