EPUB Accessibility Metadata Lint (EAA) — Licence

Free scope
  Lints the .opf file you have open and names every missing, mistyped or self-contradicting accessibility metadata value, with the exact line to paste. is free for any use, personal or commercial, for as long as you keep the
  extension installed, and the online check at the product page is free without registration.

Paid scope
  Scans every .opf in a whole catalogue folder in one run and writes a dated pass/fail report you keep and hand to a distributor. is the paid part. It asks for a licence key issued at purchase (one seat, one key). The key is checked against the payment provider and
  cached locally for 30 days so it keeps working offline.

Redistribution
  You may not resell, re-host or bundle this extension or its rule set. The rule set and the
  engine are provided as-is; check results are advisory and do not constitute legal advice.

(c) ReadyStack — https://getreadystack.com
