[css3-animations] animation-reverse: none | all | even | odd

Let me start by saying that I understand it's probably too late to 
change this, but I think the change I propose is considerably better 
than the existing syntax, so it might be worth doing so.

`animation-direction` currently [1] accepts 4 values: normal | reverse | 
alternate | alternate-reverse. Attached is a graphic that demonstrates 
all four, in a linear progression over time.

As you maybe have noticed, their effect basically is:
alternate: Reverse even iterations
reverse: Reverse all iterations
alternate-reverse: Reverse all iterations, then reverse even iterations 
again

As you can see, alternate-reverse is hard to understand, because it's 
basically a double negative. The end result of `alternate-reverse` is 
basically the same as `alternate`, with the difference that odd 
iterations are reversed instead of even ones. So, this gives us these 
four possible things we can do with `animation-direction`, respectively:
normal
reverse even iterations
reverse all iterations
reverse odd iterations

Therefore, a property that would capture this better, and would be 
easier to understand, would be called `animation-reverse` (or 
`animation-iteration-reverse` although that implies an 
`animation-iteration` shorthand which is misleading) and have the states:
none (corresponds to animation-direction: normal)
all (corresponds to animation-direction: reverse)
even (corresponds to animation-direction: alternate)
odd (corresponds to animation-direction: alternate-reverse)

Not only this is more elegant and easier to understand, but is also 
easier to extend in the future (e.g. to reverse only every third 
iteration), if such a need arises.

Thoughts?

[1]: 
http://www.w3.org/TR/2012/WD-css3-animations-20120403/#animation-direction-property

-- 
Lea Verou (http://lea.verou.me | @LeaVerou)