There have been several discussions in the EPUBCheck repo about a new validation test, which checks that the order of links in the toc nav element matches the spine order. Existing EPUBs are failing validation because of this new check, although the spec has required this since EPUB 3.0.

EPUB 3.2 spec text::

The references in the toc nav element MUST be ordered such that they reflect both:

• the order of the referenced EPUB Content Documents in the spine;
• the order of the targeted elements within their respective EPUB Content Documents.

EPUB 3.0.1 spec text:

The order of li elements contained within the toc nav element must match the order of the targeted elements within each targeted EPUB Content Document, and must also follow the order of Content Documents in the Rendition's spine.

In w3c/epubcheck#1036 there is a proposal to change this error to a warning. If we are to keep the spec aligned with EPUBCheck, that would mean changing our MUST to a SHOULD.

Should we change the EPUB spec? Is making this a SHOULD good enough?