EPUBCheck is the official W3C conformance checker for EPUB publications. It validates EPUB 2 and EPUB 3 files against the respective specifications, including OCF packaging, OPF metadata, XHTML/SVG content, CSS, media overlays, navigation documents, and accessibility metadata. The tool operates as a Java command-line application and library, using a modular checker architecture where dedicated handlers validate specific resource types (e.g., OCFChecker, OPFChecker, OPSChecker, CSSChecker). Validation is driven by a combination of RelaxNG schemas for structural rules and Schematron for cross-cutting constraints. The project includes an extensive test suite using Cucumber/BDD features and thousands of test EPUB packages covering error and valid scenarios across all profile extensions (ePub 3, EDUPUB, dictionaries, indexes, multiple renditions, previews, and region-based navigation).