Consider implementing @aria-details.
I'm not fully convinced of the utility, b/c it's more or less an on-page versus of @longdesc, and the examples included seem pretty trivial (image followed by a section describing the image).
If someone were to present a more compelling use case, or if it starts getting wider adoption, we should pick it up. Implementation would likely just be a navigation or link action from the current element to the related details element.
@aaronlev has posted the most compelling use case so far, for annotations in document suites like Google Docs. IMO, this is worth implementing now.
Related to http://trac.webkit.org/changeset/218809/webkit
Note that as part of work on ARIA Annotations, the ARIA spec is being updated to allow aria-details to support multiple ids. See w3c/aria#1136
Note the importance of aria-details has increased quite a lot with the ARIA annotations work. Supporting aria-details will be necessary in order to support comments in Google Docs, among other things.
W3C ARIA spec issue: https://github.com/w3c/aria/issues/749
Also, multiple ids are now supported. See https://github.com/w3c/aria/pull/1136
Luckily, the changeset pointed to in comment 3 already supports multiple ids.
But, I think it's only supporting ATK and not AX APIs?