We don't need a subset of HTML. Actually, we need Markdown emails. You can format stuff that needs structure, but not abusively so (no blinking marquee banners in eyesore colors), it is sufficiently compatible to plaintext that you don't even need the text alternative mime object. It is also more compact than HTML.
And before somebody says "won't fly", all those fancy new "will replace email someday" messengers use markdown or some parts of it's formatting.
And before somebody says "won't fly", all those fancy new "will replace email someday" messengers use markdown or some parts of it's formatting.