I wrote something like this when I was in high school because I got tired of powerpoint being hard to work with across platforms (Linux...). I ended up using it for a couple class presentations, which I enjoyed because it forces you to focus on content vs. unnecessary style and it's just a URL to link to instead of a maybe-compatible .pptx.

HTML based, styled with CSS. Not quite markdown, but a parser would be quick to make, I guess? https://github.com/thesephist/tesseract