RE: [css3-animations] :animating or :transitioning pseudo-class

[Tab Atkins Jr.:]
> I'm okay with something like this, but I'm curious about the use-cases.

Yes, use-cases would be very helpful here. 

If I didn't read this wrong it seems the general pattern is 'these properties 
should be set to these values but only when other properties of the element
are animating'.

If you're using :hover to trigger your transition then you don't need anything
special: just set those propertie in your :hover rule.

For other triggers - e.g. a scripted property update from a button click -
you can't really achieve that without mucking with the DOM e.g. make your button
also set an attribute to match some selector + catch the transitionend event to 
unset it. Which, given reasonable use-cases, does seem a pain.

Did I get this right?