How do I validate an EPUB file?

Use EPUBCheck: download it from github.com/w3c/epubcheck/releases (requires Java), then run: java -jar epubcheck.jar book.epub. It reports Errors (must fix), Warnings (fix before major platforms), and Info messages. Online: upload your EPUB at validator.idpf.org for instant EPUBCheck results without installing Java.

What is EPUBCheck and why is it important?

EPUBCheck is the official W3C EPUB specification validator. Amazon KDP, Apple Books Connect, and Kobo Writing Life all run EPUBCheck (or equivalent) on uploaded EPUBs. An EPUB that fails EPUBCheck with errors will be rejected by these platforms. EPUBCheck validates structural compliance: OPF metadata, manifest completeness, NCX/NAV documents, HTML conformance, and CSS restrictions.

What is the difference between EPUBCheck and Ace by DAISY?

EPUBCheck validates structural compliance with the EPUB spec — it checks that the file is a valid EPUB. Ace by DAISY checks accessibility compliance — it tests against WCAG 2.1 and the EPUB Accessibility spec (alt text, heading hierarchy, language attributes, color contrast). For EU EAA 2025 compliance and for distribution on accessible platforms, run both tools.

Does KDP require EPUBCheck validation?

KDP runs its own validation (which includes EPUBCheck-equivalent checks) on upload. EPUBs with ERROR-level EPUBCheck failures are rejected. Warnings may pass but result in lower quality scores. Amazon also checks KDP-specific requirements (cover image resolution, DRM compatibility) that EPUBCheck doesn't cover — use Kindle Previewer for those.

How to Validate an EPUB — EPUBCheck, Ace by DAISY, and Online Validators

EPUB validation catches structural errors that cause rejection on KDP, Apple Books, and Kobo, and flags accessibility issues before distribution. Here's how to validate with the right tools.

Why Validate Your EPUB?

Publishing platforms reject invalid EPUBs: Amazon KDP, Apple Books Connect, and Kobo Writing Life all run validation on upload. EPUBs that fail EPUBCheck are rejected outright.
Catch rendering bugs early: Missing manifest entries, broken internal links, and malformed CSS cause display problems on specific readers.
Accessibility compliance: The European Accessibility Act (EAA) 2025 requires accessible EPUBs for publishers in the EU. The Ace checker tests for WCAG conformance.

Tool 1: EPUBCheck (Official W3C Validator)

EPUBCheck is the official EPUB specification validator, maintained by the W3C. It checks structural compliance against the EPUB 2 and EPUB 3 specs.

Install and Run

# Download EPUBCheck from GitHub (requires Java)
# https://github.com/w3c/epubcheck/releases

# Run against your EPUB
java -jar epubcheck.jar book.epub

EPUBCheck outputs:

Errors — spec violations that will cause reader/platform failures. Must fix before publishing.
Warnings — best-practice violations. Fix these before KDP and Apple Books submission.
Infos — informational notes. Can usually ignore.

For JSON output (useful for CI pipelines):

java -jar epubcheck.jar book.epub --json report.json

Common EPUBCheck Errors and Fixes

Error	Cause	Fix
`OPF-074`	Missing required element in OPF	Add dc:title, dc:language, dc:identifier
`RSC-007`	File referenced in manifest not found	Check file paths; case-sensitive on Linux
`HTM-055`	Deprecated HTML element (e.g. <b> for headings)	Replace with semantic HTML (strong, em, h1)
`NAV-003`	Missing or empty NAV document TOC	Add nav[epub:type=toc] with entries
`CSS-017`	CSS property not allowed in EPUB	Remove position:fixed, viewport units

Tool 2: Ace by DAISY (Accessibility Checker)

Ace by DAISY checks EPUB 3 accessibility against WCAG 2.1 and the EPUB Accessibility specification:

# Install Ace (requires Node.js)
npm install -g @daisy/ace

# Run accessibility check
ace book.epub --outdir ./ace-report

Ace generates an HTML report in the output directory. Open ace-report/report.html in a browser. Key checks:

Images have alt text
Heading hierarchy is logical (no skipped levels)
Language declared in the OPF and HTML lang attribute
Tables have headers (<th>)
Color contrast meets WCAG AA (4.5:1 ratio)

Tool 3: Online Validators (No Install)

EPUB Validator (validator.idpf.org): The original online EPUBCheck interface. Upload your .epub for instant validation. Uses EPUBCheck 4.x.
Kindle Previewer: Amazon's tool validates KDP-specific requirements beyond what EPUBCheck covers. Download from kdp.amazon.com/tools.
Apple Books Automator (Transporter): Apple's submission tool runs validation before upload. Available via the Apple Books Connect dashboard.

Validating in a CI Pipeline

For automated validation on every build:

# GitHub Actions example
- name: Validate EPUB
  run: |
    java -jar epubcheck.jar book.epub --json report.json
    # Fail the build if errors exist
    python3 -c "
    import json, sys
    r = json.load(open('report.json'))
    errors = [m for m in r['messages'] if m['severity'] == 'ERROR']
    if errors:
        for e in errors: print(e)
        sys.exit(1)
    print('EPUB valid')
    "

toolkit.bot generates EPUBCheck-valid EPUBs from PDF. No manual validation step needed.

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