Back to Error DirectoryW3C EPUBCheck Standard
OPF-035_SUGCritical Ingestion Error

Use "application/xhtml+xml" instead.

Use "application/xhtml+xml" instead.

Automated 1-Click Fix Ready

Fix all OPF-035_SUG errors in your EPUB automatically

PreFlight checks for OPF-035_SUG issues in memory and provides 1-click remediation during client-side packaging.

Scan & Fix Now

What This Error Means in Plain English

EPUBCheck detected Use "application/xhtml+xml" instead.. 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-035_SUG: Use "application/xhtml+xml" instead.

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule OPF-035_SUG in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the OPF-035_SUG specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.