What EPUB 3.3 actually specifies
The specification defines authoring requirements for EPUB publications: the package, content documents, navigation, resources, metadata, security considerations and other technical details that let reading systems process the publication consistently.
Does EPUB 3.3 make older EPUB 3 files obsolete?
No. W3C states that EPUB 3.3 is backward compatible with EPUB 3.2. Moving to the newer recommendation does not automatically require authors to rebuild every sound EPUB 3.2 workflow.
What about EPUB 3.4?
EPUB 3.4 is newer work in the standards process. As of 2026 it appears in candidate-recommendation status. That matters for standards watchers, but production teams should distinguish an emerging candidate recommendation from the stable recommendation used as the normal baseline.
What authors should care about
Authors do not need to memorize the specification. They do need a file with correct structure, navigation, metadata, resources and accessibility-aware markup. Standards compliance is the foundation; reader experience is the test.
Primary sources
- W3C EPUB 3.3 — stable W3C Recommendation
- W3C EPUB standards status — current EPUB specification status