Markdown is simpler than HTML, but has more to offer than Gemini. You could have tables, images and depending on the flavor also checkboxes. Client side styling would also be easily possible. And if you want a simple browser, you don't even have to parse it and simply show the plain text.
You do not have to be a programmer to write Markdown and many writers already write in Markdown and then convert to HTML.
I'll say this: it's very cool that it's possible to make text more accessible to people using a screenreader than it is to people reading it raw. For instance, if I were to refer to the real real, I would do what I just did, and italicize the word to indicate that it's foreign. Someone who knows how Spanish works can interpolate from "italic real" and get /real/ instead of /ri:l/ out of it, and that's basically what a screenreader would do, literally say "italic real".
I'm not convinced that accessibility demands that words be pronounced properly under such circumstances. Note that someone completely ignorant of Spanish would just see "italic real" and maybe think it represents emphasis, so the sighted are not at an advantage here.
Again, it's pretty cool that there's a way to do it in HTML! Semantic markup is neat, even if it doesn't get used much.
Generally, use of this tagging is obligatory to meet accessibility guidelines. If in practice people don't use the tags so much, this is a problem. Again, the disabled should not be second-class citizens on the web or any medium that aims to substitute for it.