The FlightDeck web app extensively tests EPUB files, going beyond standard EPUBCheck errors with deeper validation and best practices clearly explained. (readioactivebooks.com)
You need to know how to run epubcheck from the command line to test this build. (safaribooksonline.com)
The generated ePub file doesn't pass epubcheck, a program that checks ePub files for compliance with the ePub standard (selfpublishingadventure.com)