Back to Error DirectoryW3C EPUBCheck Standard
HTM-061Critical Ingestion Error

"resource" is not a valid custom data attribute (it must have at le...

"resource" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).

Automated 1-Click Fix Ready

Fix all HTM-061 errors in your EPUB automatically

PreFlight checks for HTM-061 issues in memory and provides 1-click remediation during client-side packaging.

Scan & Fix Now

What This Error Means in Plain English

EPUBCheck detected "resource" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).. 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-061: "resource" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule HTM-061 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the HTM-061 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.