Back to Error DirectoryW3C EPUBCheck Standard
OPF-021Critical Ingestion Error

Use of non-registered URI scheme type in href: "resource".

Use of non-registered URI scheme type in href: "resource".

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Fix all OPF-021 errors in your EPUB automatically

PreFlight checks for OPF-021 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 Use of non-registered URI scheme type in href: "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 OPF-021: Use of non-registered URI scheme type in href: "resource".

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule OPF-021 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the OPF-021 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.