alt Hacker News> Epubcheck does basic CSS checking of course, but it can’t validate CSS against a renderer which is fundamentally broken!
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.
epubcheck is meant to ensure conformance with the standards, not the interoperably implemented subset of the standards. (Which has lots of awkward questions: which implementations of the standards, which versions of those implementations, etc.)