Back to Error DirectoryW3C EPUBCheck Standard
RSC-031Critical Ingestion Error

Remote resource references should use HTTPS, but found "resource".

Remote resource references should use HTTPS, but found "resource".

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PreFlight checks for RSC-031 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 Remote resource references should use HTTPS, but found "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 RSC-031: Remote resource references should use HTTPS, but found "resource".

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule RSC-031 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the RSC-031 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.