How Do Publishers Validate Sigil Epub For Stores?

If you're prepping an EPUB in Sigil for stores, here's my usual checklist and workflow — I treat validation like a mini detective game, hunting down HTML oddities and metadata gremlins. First, I make sure Sigil's internal structure looks sane: proper mimetype at the top of the ZIP (Sigil handles that for you usually), a valid container.xml, and a clean OPF/nav or NCX. I use Code View to quickly scan the manifest and spine to confirm every file listed actually exists and that media-type attributes are correct. I tidy up stray inline styles, remove leftover Word markup, and make sure images are RGB (stores hate CMYK) and sensibly sized — especially cover images, since Kindle and Apple do their own conversions and will sometimes complain about unusual dimensions.

After that initial sweep, I run Sigil's validator plugin (FlightCrew or the EPUBCheck bridge depending on my Sigil version). FlightCrew is great for quick checks inside the editor — it flags missing files, invalid GUIDs, and basic structural issues. But I always follow up with the official epubcheck (the command-line tool) because store submission systems generally expect EPUB to be fully compliant with the spec. epubcheck will spit out warnings and errors; errors are must-fix, warnings I triage (some are stylistic, some indicate potential reading-order or accessibility problems). Key things epubcheck catches that stores care about: duplicate IDs, broken links, missing referenced files, and nav vs NCX inconsistencies (EPUB2 vs EPUB3 issues). I keep an eye on the package metadata too — language, unique identifier (ISBN or UUID), and title/creator tags so metadata ingestion on a store side goes smoothly.

Validation isn't just technical conformance, though — I test the EPUB in multiple readers because each store's engine will render things differently. I open the file in Thorium Reader or Adobe Digital Editions for a baseline, in Readium or Firefox EPUB extensions for web-ish rendering, and then I toss it into 'Kindle Previewer' (Amazon converts EPUBs to KPF under the hood, and Kindle Previewer shows how the conversion behaves). For Apple and Kobo I use their preview tools or just load the file in Apple Books and Kobo Desktop. If something looks off — weird font fallback, CSS not applied in the right order, or TOC items missing — I trace it back to the HTML/CSS in Sigil and repair. I also test accessibility basics: alt text on images, logical heading order, and readable language tags.

Finally, after fixes and another epubcheck pass, I upload to the target store's submission portal (KDP, Apple Books, Kobo Writing Life, etc.) and preview there. Some platforms will flag proprietary issues only visible when they run conversions, so this last check is crucial. I try to keep a clean version history too — export dated EPUBs (EPUB v2 vs v3 can behave differently) so if a store rejects something I can quickly roll back. It's a bit of an art and a little like polishing a story before release, but once everything validates and previews correctly it feels really satisfying — give it a try and you might enjoy the tiny victory of a perfect validation log.