aspect-ratio property

So, we talked about aspect-ratio a little while ago.  There was
general agreement that it sounded like a pretty cool idea, but then
nobody took it farther, probably because it doesn't have a good home,
seeing as Box Model 3 is dead at the moment.

I've taken the liberty of drafting up a spec for it, for discussion
and possible later incorporation into another module -
<http://www.xanthir.com/:cz9>.

I don't have the language quite right yet, I know.  I want to disallow
it on table* elements right now, because resolution of it should be up
to the table-layout algo in play.  Generally, I just want it only
defined on things in static or positioned flow.  How do I phrase it
correctly to state that?  I also need to expand my definition of
"underspecified" to properly handle positioned elements, where
left/right/top/bottom comes into play as well.

~TJ