looking at http://www.idpf.org/epub/vocab/package/link/ it seems that saying "The media type of the record must be identified in the media-type attribute [Packages3] when this relationship is specified." for the record relation is too strong.
after all, it might be possible for a publisher to publish all record types they support at the same URI, supporting HTTP conneg.
also, clients can never depend on the URIs really returning the media type, as that may change over time, and as such any media type info is just an unreliable hint anyway: https://github.com/dret/hyperpedia/blob/master/concepts.md#target-resource-hints
what about making that a MAY and making it clear that the actual media type is a runtime issue, and that HTTP conneg can be used to communicate client expectations?