Your ePub Is fine
Points and comments are a snapshot, not live.
Kobo e-readers reject valid EPUB files due to Adobe's outdated CSS parser frozen since 2013, despite passing standard validation.
An author published an EPUB that passed epubcheck 3.3 validation but failed on Kobo devices with no error message. Investigation revealed Kobo uses Adobe's RMSDK rendering engine, originally built around 2010 and lightly updated for EPUB3 but never modernized. The culprit was a single CSS line using the min() function—valid CSS Level 4—which RMSDK's parser from circa 2013 couldn't recognize and silently rejected, crashing the entire book load.
The author identifies a core problem: epubcheck validates against standards, not against specific renderers. RMSDK lacks flexbox, grid, math functions, and custom properties, silently failing rather than gracefully ignoring unknown CSS per spec. An update notes Kobo ships a second WebKit-based renderer for .kepub.epub files (actively maintained) but routes standard .epub files to the broken Adobe engine by file extension alone.
What commenters are saying
Commenters fault Adobe and Kobo for non-compliance with CSS standards, which mandate graceful forward-compatibility: invalid properties should be ignored, not cause fatal errors. Several point out that EPUB targeting version 3.1 or 2 is more practical than 3.2+ due to the W3C's shift to WHATWG living standards. A concrete workaround surfaces: renaming files to .kepub.epub triggers Kobo's modern WebKit renderer instead of the Adobe engine. One commenter notes Adobe's RMSDK was recently sold to Wipro, though it remains outdated.