@atjn It's not about rendering -- after all, even markdown/rst is rendered into HTML and displayed using webview. The problem is that HTML+CSS is such a complex standard with such a huge attack surface, that by claiming to support the full standard you're opening up your users to huge security risks both in terms of exploits and in terms of social engineering scams.
Microsoft realized this at some point -- after a couple of years of non-stop vulnerability reports in Outlook, which is why they made a decision to not use MS Explorer to render email, but switched to using the MS Office html rendering engine, which supported only a small subset of full HTML+CSS functionality. I believe, this is largely true to this day -- if you expect your email to render properly in most clients, you can't write it to the full HTML standard, but to the MS Office stripped down set of features.
If you don't believe me that using HTML for email was a bad idea, check your incoming mail and see how many of the "rich messages" have "Email not displaying properly? View it on our website!" link at the very top of their message.