Back to Error DirectoryW3C EPUBCheck Standard
HTM-057Critical Ingestion Error

Viewport "resource" value must be a positive number or the keyword ...

Viewport "resource" value must be a positive number or the keyword "device-resource"

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Fix all HTM-057 errors in your EPUB automatically

PreFlight checks for HTM-057 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 Viewport "resource" value must be a positive number or the keyword "device-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-057: Viewport "resource" value must be a positive number or the keyword "device-resource"

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule HTM-057 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the HTM-057 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.