Re: what formats count as primary resources?

In Readium-2 we decided that universality is the right choice. As it's been
pointed out, audiobooks and comics are legitimate use case and having an
HTML wrapper is a burden, not a useful thing to have.

> if we require that each primary resource has a link to a manifest, and
that we don't want to rely on HTTP headers, that rules out images, audio,
or video

That's one of the reason why I'm against requiring a link. It's also
problematic for many other reasons including:

   - the ability to re-use the same resource in a large number of
   publications
   - creating a Web Publication with resources that you can't modify
   - it forces links to be both ways, which is never a requirement on the
   Web
   - other specifications (Web App Manifest, RSS, Atom) let the content
   creator decide if and when discovery is available, it's never a MUST,
   especially on all resources

I think that discovery should extend to HTTP and the Link header, but there
are also many situations where the author won't be able to change the HTTP
response headers for a given resource (this usually requires having a Web
App or changing the configuration of the server, none of which are
compatible with what we're building IMO).