Watch: Third-Party Email App that Renders HTML
The Apple Watch OS (watchOS) does permit rendering HTML email.  However, the Watch app must be built using watchOS 5 and use Webkit for the email.  It's also a poor experience to try to read html-formatted email on a small Watch screen.

Needless to say, most developers have not updated Watch apps specifically for this purpose and some might not ever do such an update.  

Because watchOS 5 is so new, it might take a while for developer to create such an app.  Keep checking the Watch App Store and your favorite app review sites to see if any are available or featured.