But not current web CSS, please. It manages to be simultaneously overcomplicated (including enabling fingerprinting) and really bad at laying out text (e.g. still no baseline grid). A ‘markdown web’ style sheet should be more like document processor's character/paragraph styles. It also needs to be easily overridden for accessibility or alternate presentation, particularly around size and colour (a markdown-like format should already be fine for screen readers without styling). Aside: FFS, web people, if you're setting colours at all, respect @prefers-color-scheme and do not use the inverse for code blocks.

There's also the million-markdowns problem, and markdown's HTML embedding. This being Tuesday, I'd start with djot (without embedding), but Wednesday I might go for asciidoc.

You're sort of describing Gemini.

https://en.wikipedia.org/wiki/Gemini_(protocol)

As with most things, this isn't really a technical challenge, it's a social one. The protocols you're describing already exist, more or less. No one uses them.

Why not just good old web 1.0 or even HTML5 without javascript. There are alreay plenty of pages that conform to that, you just need the client enforcement (also already available via extensions) and marketing/lobbying so that big organizations switch to it.