tl;dr: An author's EPUB passed epubcheck validation and worked on Kindle, Apple Books, and Thorium, but failed silently on Kobo devices. The culprit was a single line of modern CSS (`max-width: min(150px, 30vw)`) that Adobe's RMSDK rendering engine—frozen circa 2013 and used by Kobo for standard EPUBs—couldn't parse, causing it to drop the entire book instead of ignoring the unknown declaration as the CSS spec has required since 1996. Workaround: rename files to `.kepub.epub` to route them to Kobo's actively maintained WebKit renderer instead.