Alex,
 
You can't expose aria-details as an action it is a relationship to the extended detail. Who said you were to implement an action? It is not a longdesc.
 
It could point to a details/summary which shows and hides the content itself. The target might be hidden by book readers until the user turns on a facility to show the extended description.
 
 
Here is an example:
 
<img alt="foo" aria-details="baz" src="">
 
<details id="baz">
<iframe src=''m not yet certain if we agreed upon how to expose aria-details, because longdesc is exposed as an action, which pops up a window with the longdesc, when invoked. My understanding is it is a web author responsibility to implement aria-details UI, and thus action mechanism perhaps is not suitable for it.
 
Jamie, are we going with a new relation for aria-details or can you think of another mechanism to expose it?
 
Thanks.
Alexander.