Validation of meta elements in EPUB3 triggers a few surprises... In fact, that's not the meta elements themselves, it's the properties they carry. Let me explain:

  • EPUB3 meta elements carry a property attribute
  • a property is a keyword optionally preceded by a property vocabulary prefix and a colon
  • EPUB3 Media Overlays use the reserved media prefix mapped to namespace http://www.idpf.org/epub/vocab/overlays/#
  • the Media Overlays says the media:duration property has a cardinality of "exactly one for the Publication and for each Media Overlay"

In fact, that's quite painful and costly to test in the following case:

  1. suppose the namespace above is declared (yes, I know it's already reserved but it's not forbidden to do it !!!) on the package's prefix attribute with prefix foo
  2. suppose we have one <meta property="media:duration"> in the OPF for the Publication
  3. but also a <meta property="foo:duration"> for the same value...
  4. that's invalid per spec, see the last item of the list above

You can't compare the prefixes, that's not enough. You really need to rely on URIs and that's where validation is expensive.

Conclusion: Media Overlays 3.0 have been released with EPUB3. The associated media prefix should be dropped and its related properties should be prefixless in EPUB3. If the issue is the media:narrator property too close to a value in another property vocabulary, it should be changed.