Are there potential mappings for anchor rel values?

Inspired by #613, I'm wondering if AAPIs might be able to make use of some rel attribute values on a elements. The HTML spec's link type list is fairly small (for a) and potentially useful values might be fewer still.

But e.g. external could help identify external links programmatically (and more reliably than AT heuristics around href values). I could also see some use for bookmark (e.g. for self-referencing links in headings).