As someone who works on an email system, it's a bit of a shame that this didn't win over HTML at least for email between humans. HTML is a bit of nightmare to sanitize. It's also expensive to layout compared to something like text/enriched which wouldn't need a full WebView on mobile - you'd be able to easily layout using native view widgets.
Especially when you consider HTML in emails is stuck on a mostly old school table based layout system for highly styled emails, and a lot of these emails are not responsive.
I remember seeing this and liking it. It is too bad that it did not, apparently, catch on. I seem to recall suggesting to the RFC authors that the RFC itself should be available in text/enriched.
Emacs has a mode for this: <a href="https://www.gnu.org/software/emacs/manual/html_node/emacs/Enriched-Text.html" rel="nofollow">https://www.gnu.org/software/emacs/manual/html_node/emacs/En...</a>
Refusing to use attributes in this kind of *ML and using <param> instead makes this a mess to parse safely. I imagine this will be read as HTML and the params will show up in the document oddly.
Always wondering if there's a standard format for the thing in emails that uses underscore, slashes, asterisks for underline, italics, bold and less/greater than for URLs. Never found it. Always thought it would be a great addition to pandoc output formats :)