Back to Error DirectoryW3C EPUBCheck Standard
OPF-090Critical Ingestion Error

It is encouraged to use MIME media type "resource" instead of "reso...

It is encouraged to use MIME media type "resource" instead of "resource".

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Fix all OPF-090 errors in your EPUB automatically

PreFlight checks for OPF-090 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 It is encouraged to use MIME media type "resource" instead of "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-090: It is encouraged to use MIME media type "resource" instead of "resource".

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule OPF-090 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the OPF-090 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.