Your EPUB passes every validator. Kobo still chokes on it. The culprit is from 2013 (andreklein.net)
A finished EPUB validates fine, then renders as a blank or broken page on Kobo with no error. The cause is RMSDK, Adobe's proprietary engine inside Kobo, built around 2010 for EPUB2 — no flexbox, no grid, no CSS math functions. Modern CSS doesn't degrade, it crashes.
An indie who tried to license RMSDK directly: the email on Adobe's site answers nothing, no internal docs exist, the product was quietly sold off. Kobo is now rewriting its reader to drop Adobe entirely.
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