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EPUB · QUALITY VALIDATION

An error code is not yet a solution.

EPUBCheck validates the technical structure of an EPUB file. What matters is not merely that a message appears, but which resource is affected, why it happens and which production step truly resolves it.

Editorial team: epubio · Responsible editor: Daniel Trogisch

IN BRIEF

What EPUBCheck actually tells you

EPUBCheck compares package data, content documents, navigation and references with the rules of the EPUB standard. Passing is an important technical prerequisite, but not a guarantee of store acceptance, good typography or complete accessibility.

Read each finding in this order: severity, message identifier, affected file, line or location, and content context. Resolve errors first, then assess warnings, and validate the newly generated file in full.

STEP BY STEP

From validation report to dependable correction

Do not work down the report at random. One early structural cause can trigger many later messages.

01

Record the validated book state

Note which EPUB file and book state were checked. If manuscript, cover or metadata changes afterwards, the old result no longer describes the current edition.

Check this
  • Record filename and generation time
  • Do not pair an old file with a new report
  • Associate changes with one book state
02

Read severity first

An error normally identifies a rule violation that must be resolved. Warnings and informational messages require context: they can flag risks or recommended improvements without making the publication inherently invalid.

Check this
  • Address errors first
  • Do not dismiss all warnings
  • Assess informational findings in context
03

Combine message and location

The identifier names a class of finding; the path and line show its concrete location. Read them together. A navigation message may originate in a missing or misnamed content target.

Check this
  • Keep the message identifier
  • Open the file path and line
  • Inspect both reference and destination
04

Fix the earliest shared cause

Several messages can result from one broken link, an invalid media type or incomplete metadata. Correct the source in the book project rather than merely editing the exported ZIP package.

Check this
  • Group similar messages
  • Repair the source before consequences
  • Avoid cosmetic patches to the finished EPUB
05

Generate the entire EPUB again

Create a new edition after the correction. Replacing individual files inside the archive can make the manifest, checksums or internal references inconsistent again.

Check this
  • Generate from corrected source
  • Archive the old file clearly
  • Avoid partial repair without specialist review
06

Revalidate and perform a visual review

A clean technical report does not replace reading. Open the EPUB in suitable reading systems and inspect navigation, chapter openings, images, links and typography.

Check this
  • Run EPUBCheck again in full
  • Review in more than one reading context
  • Keep report and file on the same state

DIAGNOSTICS

Common groups of findings in plain English

Finding patternTypical causeUseful correction
Finding patternA referenced resource cannot be found.
Typical causeFilename, letter case or relative path does not match the actual target.
Useful correctionInspect reference and target together, then generate the EPUB again.
Finding patternAn element or attribute is not allowed here.
Typical causeThe XHTML uses a structure that is invalid in the detected context.
Useful correctionSimplify or correctly mark up the affected content in the editor and re-export.
Finding patternManifest entry or media type is inconsistent.
Typical causeA packaged file is missing from the manifest or declared with the wrong type.
Useful correctionInspect the source asset in the project and rebuild the complete export.
Finding patternNavigation or a content destination is incomplete.
Typical causeA chapter lacks a valid destination, an ID is missing or an entry points nowhere.
Useful correctionReview chapter structure and headings, then regenerate navigation.

SOURCES & LIMITS

Primary technical sources

W3C · EPUBCheckOfficial project page for the EPUB validator, current releases and documentation.Open primary sourceW3C · EPUB 3.3Normative specification for the structure and behaviour of an EPUB publication.Open primary sourceW3C · EPUB Accessibility 1.1Requirements and conformance for more accessible EPUB publications.Open primary source

Useful next steps

Validate EPUBThe guided epubio check with a reason and next action.Create EPUBGenerate a new edition from a controlled source.MethodologyWhat passed, notice and blocker mean in epubio.ReleaseTwinSee when a validation result no longer belongs to the current book state.

BOOKPASS

Validation is complete only when the next action is clear.

epubio connects EPUBCheck findings to the affected part of your book project.

Explore BookPass