The first thing to bear in mind is that HTML is converted to Markdown for posting. Not all HTML converts reasonably to Markdown.
On top of that, HTML standards are all over the map for email, so it’s indeterminate – depending on your mail client that may not render at all. It would be nice if you could assume
recent email client = current-ish version of modern web browser
but that is almost never the case. Even with Gmail there are significant limitations, that is why services like https://litmus.com/ exist and are critical.