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HTM-059Critical Ingestion Error

Viewport "resource" property must not be defined more than once, bu...

Viewport "resource" property must not be defined more than once, but found values [resource].

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What This Error Means in Plain English

EPUBCheck detected Viewport "resource" property must not be defined more than once, but found values [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 HTM-059: Viewport "resource" property must not be defined more than once, but found values [resource].

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule HTM-059 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the HTM-059 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.