You might wanna look into Gopher and Gemini protocols, as they seem to be pretty much exactly that

Gemini is probably not exactly what GP wished for, but it has something resembling a critical mass of users and while I do not think the text format (gemtext, i.e. gmi) is perfect I find it good enough for what it does.

I even use gemtext now and then offline just as an even simpler markdown. Since it has so few features it is trivial to convert gmi to markdown or to any other format without losing anything. It works as a lowest common markup language for when something that minimal is enough.