The old IDPF conformance URLs have to be updated for the next release and we also need to figure out how to ensure a more stable reporting method going forward.

The base URL to use for the epub accessibility specification should be fairly straightforward as we'll have a versioned /TR path to use this time (no need for a dated specification).

The complexity comes in also having to report the WCAG version and level, assuming we are allowing flexibility in conformance. We should figure out how important this information is to break out. Do vendors care about the specifics of conformance, for example, or only that content conforms to the accessibility standard?

In a world where the details aren't that critical, one option would be to drop the WCAG conformance level from the conformsTo URL entirely so only the accessibility specification URL is used. The WCAG version and level could be required in the accessibility summary, for example, so users can still understand the quality. For example:

<link rel="dcterms:conformsTo" href="https://www.w3.org/TR/epub-a11y-11/"/>
<meta property="schema:accessibilitySummary">... This publication conforms to WCAG 2.1 Level AA. ... </meta>

One problem with this approach, however, is that the information won't be easily parseable by tools like Ace. But these tools may also always assume conformance to the latest version (making them account for variances between releases could prove a maintenance headache).

Alternatively, we could require (or recommend) a second conformsTo field that specifies the WCAG version:

<link rel="dcterms:conformsTo" href="https://www.w3.org/TR/epub-a11y-11/"/>
<link rel="dcterms:conformsTo" href="https://www.w3.org/TR/WCAG21/"/>

The problem remains here is that WCAG doesn't have URLs for each conformance level, so unless we require AA conformance we're still going to be missing information. We could require that context in the summary, though.

Looking at the Dublin Core definitions again, we might also simply consider using a natural language descriptor for WCAG conformance. The vocabulary now says this:

Creators of non-RDF metadata can use the terms in contexts such as XML, JSON, UML, or relational databases by disregarding both the global identifier and the formal implications of the RDF-specific aspects of term definitions. Such users can take domain, range, subproperty, and subclass relations as usage suggestions and focus on the natural-language text of definitions, usage notes, and examples.

https://dublincore.org/specifications/dublin-core/dcmi-terms/#section-1

This sounds to me like the rigidity of dcterms properties for linked data has been relaxed. In that case, we could require authors to follow a pattern like "WCAG 2.X Level AA":

<link rel="dcterms:conformsTo" href="https://www.w3.org/TR/epub-a11y-11/"/>
<meta property="dcterms:conformsTo">WCAG 2.1 Level AA</meta>

We could also take this one step further and incorporate everything in one common pattern like:

<meta property="dcterms:conformsTo">EPUB Accessibility 1.1 + WCAG 2.1 Level AA</meta>

That's as far as my thought experiments have taken me with this, at any rate. Other ideas welcome.

/cc @avneeshsingh @clapierre @GeorgeKerscher