We corrected that reports need to be associated with the certifier in #1410, but we don't indicate who has certified the conformance claim. We should similarly show an example with the certifier linked:

<meta property="dcterms:conformsTo" id="a11y-conf">EPUB-A11Y-11_WCAG-21-AA</meta>
<meta property="a11y:certifiedBy" refines="#a11y-conf">Acme Certifiers</meta>

The problem is we'll introduce warnings/errors if we require this normatively, so it has to only be an example.

I wonder if it would make sense to say refining is optional if only one conformance claim/certifier is included in the metadata, but otherwise should be used? (But also keep this an informative statement.)

It is needlessly cumbersome unless there are multiple conformance claims or certifiers.