reversing of transitions

I finally edited the rules for reversing of transitions into the
transitions spec:
https://dvcs.w3.org/hg/csswg/rev/9eb0718d6baa
http://dev.w3.org/csswg/css-transitions/

Since we discussed in Paris that you'd implemented the new proposal,
I'd appreciate if you could review the edits, both to see if they
make sense and whether they match what you thought the proposal was
and what you implemented.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)