Back to Error DirectoryW3C EPUBCheck Standard
OPF-073Critical Ingestion Error

External identifiers must not appear in the document type declaration.

External identifiers must not appear in the document type declaration.

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Fix all OPF-073 errors in your EPUB automatically

PreFlight checks for OPF-073 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 External identifiers must not appear in the document type declaration.. 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 OPF-073: External identifiers must not appear in the document type declaration.

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule OPF-073 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the OPF-073 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.