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