An author discovered that EPUB files passing epubcheck validation can still fail silently on Kobo devices due to Adobe's outdated RMSDK rendering engine. Despite using perfectly valid CSS level 4 syntax, books crash without error messages on Kobo hardware that relies on this legacy Adobe technology.

The culprit was a single CSS line: `.copyright img {max-width: min(150px, 30vw);}`. While this represents modern, standards-compliant code, RMSDK's CSS parser—frozen around 2013—doesn't support mathematical functions or newer layout features. The author tested dozens of variations before identifying this specific incompatibility.

epubcheck validates against EPUB standards but cannot account for fundamentally broken renderers like RMSDK. This creates a dangerous gap where technically perfect ebooks become unreadable on popular devices. Publishers must now test beyond validation tools, running files through Adobe Digital Editions to catch silent failures.

The issue extends beyond Kobo to older Sony and Nook devices sharing the same Adobe backbone. For developers distributing EPUB content, this means maintaining compatibility with decade-old rendering engines despite using valid modern web standards—a frustrating compromise that prioritizes DRM enforcement over reading experience.