EPUB 3.3
EPUB 3.3 is the current version of the EPUB standard, maintained by the W3C, that defines how an ebook file is packaged. An EPUB is essentially a zipped bundle of web content — text, images, styles, and navigation — plus metadata that tells reading apps how to display the book.
Validation matters because retailers run automated checks on every file they ingest. EPUBCheck is the W3C's open-source validator: it confirms the package is structurally sound, the navigation works, and the metadata is well-formed. A file that passes EPUBCheck is far less likely to be rejected or to render badly across the wide range of e-readers and apps.
For indie authors, the practical takeaway is simple: whatever tool produces your ebook, the output should be a valid EPUB 3 file. Word processors and ad-hoc converters often produce files that look fine in one app and break in another; a validated EPUB behaves predictably everywhere. Scribegrove's Publishing Studio exports EPUBCheck-validated EPUB 3.3 files directly from your manuscript.