A naive reading of the spec doesn't really give a true raison d'être for its existence. The spec is a little hand-wavy about required resources and fonts, etc. but it doesn't prove that current web technologies don't already do everything described.
What would be great would be more clarity about that. That is, a really clear, technical proof, that "today, the Web cannot do books: _why this spec is really needed here." It seems like DRM would be the only thing (?), as I'm having a hard time thinking of what can't be built today using Web tech with regards to "books" on the Web - and hypermedia in general, which the Web is pretty good at.
Let's frame this differently: let's say you came to Mozilla, Google, or Apple and asked them to implement the spec. What would you want the browser to do differently and why? And how would that be different to "web apps"?
As a web developer, I can already do offline with Service Workers, etc. The fonts issue can also be handled through the fonts API and through the cache API, fetching is handled by the fetch spec. And so on... and the merging of data is trivial too (e.g., Object.assing({}, JSON.parse(a), JSON.parse(b)))
). so, it would be great to identify what the actual gaps are that the spec is trying to standardize.
It might be that the Web provides all of what is needed already? It be really cool to do a rundown and see what is missing.