Re: transitions vs. animations

Simon Fraser wrote:

 > Transitions provide interpolation between existing CSS property
 > value changes. In other words, if the 'left' property changes from
 > 10px to 100px, without a transition it will change instantaneously.
 > With a transition, it will change over time. This allows
 > transitions to have a nice fallback behavior in older browsers
 > which don't support them; users won't see the nice smooth changes,
 > but the changes still happen.
 > 
 > The key concept with animations is that you apply style in order to
 > put an element into an animating state. Just as you use CSS to set
 > the highlight state of a button, you can use CSS to put an element
 > into a "bouncing" state. That state may be temporary (finite number
 > of iterations) or permanent (infinite). The properties in the
 > animation keyframes are applied after all other properties, and
 > override them, so animations don't provide nice fallback behavior
 > (older browsers simply won't show the animations).
 > 
 > I think these concepts are different enough to warrant keeping them
 > separate.

I agree that having a fallback (transitions) or not (animations) is a
difference. But I'm unsure if that difference warrants one extra
specification and another set of properties. And I don't think the
"states" are fundamentally difference -- the "states" in transistions
just seem to be a subset, or a cousin, of the states in "animations".

A better reason for having two different specs/property-sets would be
if it makes sense have both on the same element. I.e., are there
any use cases where you would set both a transition and an animation
on the same element?

Here's a list of properties that seem to have the same function:

 transition-duration
 transition-timing-function
 transition-delay

 animation-duration
 animation-timing-function
 animation-delay

So, we could at least save three properties (and probably a shorthand
property, too) if the concepts were combined. This makes it worth
investigating. And, don't forget my headache :)

-h&kon
              Håkon Wium Lie                          CTO °þe®ª
howcome@opera.com                  http://people.opera.com/howcome