The best ebook format I have ever experienced is .txt and just let the software figure out where the text needs to go.
But isn't that kind of the point of epubcheck? It's surely not intended to validate all of CSS, it's intended to validate that an epub will work... and not working on Kobo devices (probably #2 manufacturer of ebook readers?) is a major issue.
The standard exists, it is the responsibility of both the producer and consumer of ePUB files to adhere to the standard.
Though these days I have to spend more time worrying about EAA and ADA compliance than anything else.
These days I usually get 90% of the way on google docs, then do the final editing on LibreOffice which can add things like tables of contents and cover image, if it opens on Kindle, Kobo and Calibre I consider it job done.
I can't tell what the writing or design are like, because the article renders as a blank page on my old iPad.
Also on my 2026 Kindle Paperwhite.
It's futile to be old man yelling at cloud on HN, but I'm still irritated by web pages that are essentially text, yet can't be bothered to actually display anything. A blank page.
Presumably ePub publications are easier to QA than web pages. They must conform to a subset of web standards.
(FWIW, ADE will probably die when Rosetta support goes away with MacOS 28, but one of the de-DRM plugins will read acsm files directly and bring in the books.)
(Others point out that Calibre automatically will rename epub files to .kepub.epub for you if you use it to manage a Kobo library. It's just manually copying files to Kobo where you need to remember to do it yourself if you have a Kobo.)