media-type requirements on link

[mattgarrish]

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Epubcheck always emits an error on link rel="record" if the media-type attribute is missing, but we changed the requirement for 3.1 so that media-type is only required for resources in the container. See https://github.com/w3c/epub-specs/issues/702

The spec wording is now:

The media-type attribute is OPTIONAL when a linked resource is located outside the EPUB Container, as more than one media type could be served from the same URL [URL]. EPUB Creators MUST specify the attribute for all Local Resources.

If you specify any other rel value, epubcheck doesn't appear to require media-type on resources in the container, but should per this definition.

[mattgarrish]

FYI, I just made a test to check if a local resource would get flagged (in the testing branch), but there isn't an error in the beta-2 release.