Regarding these CSS3 pseudo-elements:
They are mentioned in this W3C document from 2004:
But I can't read any reference to them in in later years.
I think they are to do with XForms - whatever they are.
Have they now been abandoned?
I haven't heard anything about them..I don't think the w3 is focused on them at the moment but I don't think they have b een abandoned since that is the most recent version of document
They haven't been abandoned, as Ryan said their in the most recent version of that part of the CSS3 specification (UI modules).
What you need to understand is that the W3C works very slowly, so they probably haven't gotten any further yet (and browser support takes time).