Back to Error DirectoryW3C EPUBCheck Standard
CHK-003Retailer Warning

Unrecognized custom message severity "resource" encountered in mess...

Unrecognized custom message severity "resource" encountered in message overrides file "resource".

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Fix all CHK-003 errors in your EPUB automatically

PreFlight checks for CHK-003 issues in memory and provides 1-click remediation during client-side packaging.

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What This Error Means in Plain English

EPUBCheck detected Unrecognized custom message severity "resource" encountered in message overrides file "resource".. Digital reading systems and bookstores require standard conformance to ensure compatibility across all reading apps.

Why Retailers & EAA Regulations Block This File

Automated book distributors (Apple Books, Amazon KDP, Kobo, and IngramSpark) run strict EPUBCheck validation on ingest and reject non-compliant files.

Technical Specification: W3C EPUBCheck rule CHK-003: Unrecognized custom message severity "resource" encountered in message overrides file "resource".

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule CHK-003 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the CHK-003 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.