Why? People who like HTML emails are happy with it; people who prefer plain-text emails (like me) won't like your solution either, since it's XML.
I think we already have a decent format for lightly formatted email, and it's the plain-text conventions (in which Markdown was based). Or Markdown itself, if you want something closer to a standard.
Plain text should stay as is. HTML is the problematic format that needs addressing. When they first started using HTML for e-mails it was just the natural thing to do, but with the benefit of hindsight it was a mistake.
So I am suggesting we take a subset of HTML, add a very small set of e-mail specific features (signatures, quoting, etc) and then deploy. This format should be entirely display-able with a single XSL (as the easiest implementation).
I disagree: Libraries to render HTML already exist and will continue to be developed, since they are used in web browsers anyway. The development effort would be greater if we introduce another (new) format specifically for email. Every E-Mail Client that wants to do so already implements HTML display.
The problem with HTML mail in my opinion is simply that it is overused. Often it is unnecessary bloat, and plain text works just fine.