Your ePub Is Fine. Kobo Disagrees. Blame Adobe

An author discovered that their ebook, which passed all standard EPUB validation checks and worked on multiple platforms, was rejected by Kobo devices because Kobo uses Adobe's outdated RMSDK rendering engine that doesn't properly support modern CSS stylesheets. After extensive debugging, the author traced the issue to a single line in their book's stylesheet that Adobe Digital Editions couldn't process, highlighting a compatibility gap between the EPUB standard and Adobe's proprietary software that powers Kobo's devices.