Remove requirement for page list to match the order of page break markers

[mattgarrish]

As currently recommended in #1500, this PR removes the requirement for the page list to match the order of page break markers in the content.

Assuming this is accepted by the WG, a future PR will address recommending the page list match the print sequencing in the accessibility spec.

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[iherman]

The issue was discussed in a meeting on 2021-04-01

List of resolutions:

• Resolution No. 2: Merge PR 1588

View the transcript

3. Pagelist Requirements

See github pull request #1588.

See github issue #1471.

Dave Cramer: we had required that the order of the nav elements had to match the order of the elements in the spine
… then when this was implemented in epubcheck, it turned out that it made a bunch of epubs invalid
… we eventually got that sorted out, but we never went back to address the corresponding issue in the pagelist
… mgarrish is this going to be covered in epub a11y instead of core now

Matt Garrish: yes. The issue is that this forces the pagelist to match the order of the digital version, which is not at all helpful when there is an expectation that the pagelist correspond to print

Brady Duga: why are we removing this requirement?
… the toc is not without controversy because it breaks UIs
… we didn't want to make publishers go back and fix their content
… is this similar?

Matt Garrish: here we're just removing the strict requirement that the pagelist match
… so we wouldn't be invalidating anything
… not the same as the toc because the toc is being used by RSes in various ways
… but we don't know that RSes are using pagelist in the same way
… allowing authors to reorder pagelist might actually make it align with expectations

Brady Duga: so, does this mean, for e.g., that page 7 could come after page 10?

Matt Garrish: yes, but you could still do that right now if that was the order of content in the spine

Wendy Reid: with the current requirements we might accidentally be forcing the page 7 after page 10 thing
… with the PR that might still happen, but it would be by choice, as opposed to being forced by spec

Ben Schroeter: you could also have a case where the print book has a sidebar that in the digital version comes after the body text
… in cases like that you might want the pagelist to come out of order intentionally

Matt Garrish: i'm not sure if that would be an example of where we'd want that for a11y purposes or not, but there are situations where you'd want the pagelist to match print content rather than spine order

Dave Cramer: i'm seeing several cases where we're making the choice that these "best practices" are better off being in the a11y spec when the directly impact a11y rather than try to bake good practice into the epub spec itself, especially when there is such a diversity of content

Ben Schroeter: but i've always wonder why pagelist is an a11y feature
… it is, but it is also useful for everyone

Matt Garrish: that's a possibility we could take up, i.e. just recommend something about page sequencing

Proposed resolution: Merge PR 1588 (Wendy Reid)

Ben Schroeter: +1

Matthew Chan: +1

Matt Garrish: +1

Shinya Takami (高見真也): +1

Wendy Reid: +1

Dan Lazin: +1

Toshiaki Koike: 0

Brady Duga: +0 as I don't fully understand all the implications

Masakazu Kitahara: 0

Resolution #2: Merge PR 1588

Wendy Reid: i understand your concern duga
… and this is something we can take a look at when we do virtual locators as well
… as there is crossover

Brady Duga: it may be fine, but i'd just like to think about it some more first