Based on the discussion, the two areas of greatest
From the EPUB3 spec:
The spine element defines the default reading order of the EPUB Publication content by defining an ordered list of manifest item references.
The spine represents an ordered subset of the Publication Resources listed in the manifest, with content items not being referenced being ancillary to those that do.
Reading Systems must provide a means of rendering a Publication in the order defined by the spine, which includes: 1) recognizing the first primary (linear='yes') item in the spine as the beginning of the main reading order of the Publication; and, 2) rendering successive primary items in the order given in the spine.
So a component that isn't in the spine is "ancillary" to the main content. This is interesting, because no such burden is placed on nav. In fact, for some FXL children's books, our nav file consists of a single entry, labelled "begin reading." That label reminds us there is text associated with each entry in a nav element, the title of the thing you're pointing to. So we have one thing (spine) that is required to point to all the "primary" components of the document, but has no obligation to name those components. And we have a different thing (nav) that is required to name any component it points to, but is (mostly) optional.
EPUB Zero proposes that index.html contain at least one nav element, which would identify the primary reading order of a document based on the content documents (not necessarily html) referenced.
So in some sense we're requiring the main navigation document to identify and label every component of the primary reading order, which is a greater constraint than in EPUB3. Could this raise the baseline for accessibility?
If we do this, the primary navigation document could not refer to files that are not in the linear reading order. Is this too high a price to pay? Is there a class of documents that aren't important enough to be in the main flow, but too important to omit from the primary nav file? I believe I could happily live with such a restriction, but I wonder what others think. [keep in mind there would be no restriction on linking to 'out-of-spine' content].
If we separate the functions of navigation and reading order, how would we describe the reading order using only HTML5? If there is a "navigation" nav file, and a "reading order" nav file, how do we distinguish them?