I don't think everyone will like markdown or any html-free subsets of it. It'd be better to have a low level format for documents which could be a text or a binary format.
If I think about a text format, the first thing that comes up in my mind is something like Scribe or Texinfo.
Like this:
This is what you write...
---
title = Title here
---
# Heading here
Blah **blah** _meh_ duh @haha.
which is transformed into something like this...
@document[type=lwdml,version=1.1]
@meta[
generator="lwdml-md 0.2.7";
time='iso8601#2021-12-31T23:57:34Z#';
docversion=0.1;
]
@title|Title here|
@main
@heading(Heading here)
@start{text}
Blah @bold(blah) @emph(meh) duh @@haha.
@end{text}
@eof.
...which is then served. This is at least human readable, clean and less repetation and clutter (than html (I think)).
In this system, you'd be writing in your favourite markup language (markdown, reST, ...) and compiling it to be served :).