The MO spec currently says in 3.2.1 Structure.

The ordering of [Media Overlays] elements MUST match the default reading order of the EPUB Content Document.

Can this conformance requirement be clarified? Should it be moved to the accessibility spec instead?

Someone from a DAISY member organization recently brought up to us an interesting use case: an old Chinese poem where the reading order differs from the visual order. In display, the text has three paragraphs A, B, C. The reading order is A, C, B.

The issue with the requirement above is that "reading order" isn't defined anywhere.

As @mattgarrish said (in an internal discussion around this):

Looking at it again, the wording is kind of ambiguous, as it doesn't say it must match the order of elements in the file like we had for the table of contents. I suppose you could read the requirement that if the default reading order is not sequential then the order of the media overlay also must be non-sequential
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The accessibility requirement for elements to be ordered as you'd expect to read them is a content issue.

@marisademeglio then pointed out that this is somewhat linked to WCAG SC 1.3.2 which says:

When the sequence in which content is presented affects its meaning, a correct reading sequence can be programmatically determined

In my understanding, and to simply, the DOM order provides the reading order. For the use case above, a solution would be to have the DOM and MO respect the meaningful reading sequence, and to use CSS to visually re-order the paragraphs.

@marisademeglio says that footnotes are an interesting use case too. They can be at the bottom of a text document, but inline and skippable in the SMIL file.

I'm hoping we can clarify all this in 3.3.