Comment by mannyv

"Epubcheck does basic CSS checking of course, but it can’t validate CSS against a renderer which is fundamentally broken!"

According to the author, Kobo uses CSS from 2013. A quick check with an AI says RMSDK supports CSS 2.1 and parts of 3.

So it's not that the renderer is broken, it's that he believed that epubcheck actually checks against devices and the versions of CSS that those devices support.

This is exactly the issue with test tools: the test tool tests to a spec, but the platform is the gold standard. If you don't like it tough shit.