Gemini sets a good example, but what I’m specifically advocating for is an incremental improvement to how we use the existing web. Fixing e.g. [1] (falling back to plaintext) should be trivial, and with that obstacle removed there would be nothing stopping early adopters from publishing in markdown today.
Perhaps advertise to the browser per se as plaintext, then have an extension look for a magic number or readable flag to indicate specific-markup-type, and translate from there more opportunistically.
Plaintext infrastructure; readable markup; optional (semi-automated?) styling from there.
(Perhaps your higher-up comment already suggests as much.)
I think you can just serve text/markdown and things that don't know the markdown subtype will just display as plaintext because of the text main type while things that do e.g. extension registering the mime type will render it. Would need to test to verify though.
[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1319262