I've been building SmallDocs [1], a fairly fully functional office suite (text, formatting, presentations, spreadsheets) which is based in markdown (and mainly written by agents).
Given my experience building this, I think the idea of "a truly independent document":
one that displays identically on any system, in any country, for any user, regardless of the software used
Will be very difficult to achieve in practice.
Building software is an expression of opinions. And one of the main drivers of building an app is to innovate or improve the user experience vs. what exists today. These things combine to result in new features, and new features breaks the open standard.
If I wanted to build a new Excel, I might want to add something to spreadsheets which a basic open source reader cannot, or even Excel itself, might not be able to handle.
With SmallDocs, I built on the standard rendering of markdown, but then added a bunch of properties which traditional markdown renders would ignore.
For example, styles front matter:
---
styles:
fontFamily: Lora
baseFontSize: 17
h1: { fontSize: 2.3, fontWeight: 700 }
p: { lineHeight: 1.9, marginBottom: 1.2 }
...
---
And slide blocks like:
~~~slide
grid 100 56.25 bg=#0b1220
r 6 9 82 4 text=caption color=#fbbf24 align=left | LAUNCH REVIEW - Q2 2026
r 6 18 84 16 text=title color=#f8fafc align=left | Atlas 2.0 shipped to every customer
r 6 37 78 6 text=subtitle color=#94a3b8 align=left | What changed, what it cost, and what we learned
l 6 47 24 47 stroke=#fbbf24 strokeWidth=0.2
~~~
All this means SmallDocs markdown files only render correctly through our rendering engine - but this enables me to add something new to an existing document format.
[1] https://smalldocs.com; https://github.com/espressoplease/smalldocs