Originally Posted by Hadrien
Should be allowed as long as you've set up a DTBook or XHTML flow as the fallback.
Actually, I'm not sure the OPF spec allows that. The only reference it makes to an item with one of the OPS Core Media Types having a fallback is:
For the purpose of fallback specification, the file with the media type application/x-dtbncx+xml that specifies the publication’s NCX (see below) should be considered a Core Media Type, thus fallback information must not be provided for this file.
fallback information must not be provides" bit is what gets me. Instead of making NCX a special exception which must not have fallback information provided, the spec makes it one of the Core Media Types and implies that because of this
it is disallowed from having a fallback.
Is disallowing such fallbacks actually useful? I can't think of a good reason for it.