AFAIK, "email HTML" isn't standardized either; most organizations that make nice-looking HTML emails have to do a ton of testing across different clients and come up with workarounds to make everything look consistent.
HTML 2 might be an interesting subset of HTML to "focus on" for smolweb, but it would be a big retro throwback, and not feel at all modern.
If you were starting today, might be more interesting to start with the most modern stuff and work backwards. HTML 2 TABLE could be implemented as a specialization of CSS Grid, for instance.
If it actually gets mainstream adoption or goes into the standards pile it another question entirely.
Your standard still needs to render in Outlook on Windows, though, which means you need to support the weird Office version of IE11 as an upper limit.