EPUB can't cover this niche.

EPUB is basically just HTML in a zipfile, with all the ugly warts that HTML comes with. (Attempts to enforce "pixel-perfect" design, broken accessibility, ugly fonts forced on you, etc.)

EPUB is good for publishers, I guess, but a crappy ebook format.

FB2 is a different beast, it's an extremely minimal XML dialect that encodes only semantic information and nothing else. (Basically, tags for chapters/sections/headings/verse and that's about it. The extent of the formatting allowed is 'em' and 'b' tags.)

You can create your own clean minimalist epubs. None of that cruft is mandated, you can simply choose to not use any of it. Don't want to enforce a font? Don't specify one. Don't want a stylesheet? Don't put one in.

Presumably the problem here is restraint? I contribute to Standard Ebooks that uses EPUB3 as a core format. That’s based on HTML5.2 but our books have clean code, minimal styling and as good accessibility as we can make (we’ve had a couple of reviews of the core accessibility functionality and fixed any problems that came up).