Note by @fastfinge

@WeirdWriter @skye The issue with HTML is that a lot of browsers these days will refuse to download HTML, and will just open it. For less techy people, actually saving an HTML file they got from a link can be challenging. And markdown is fine on desktop, but I'm not aware of anything on mobile that allows navigating by headings in a markdown file. Personally I'd say epub. There are free programs on all the platforms that can open epub, browsers just download it by default, making accessible epubs is as easy as making accessible html, and it will work for blind and sighted alike so you don't need to create two different documents.