when do transitions occur?

Hi folks,

I have a question around Transitions/Animations. When exactly do transitions/animations occur? I'm a bit unclear on this point. Do they occur if delay and duration are both 0s?

The wording in the spec ("By default the value is '0', meaning that the transition is immediate")  seems to suggest that a transition/animation does occur even if duration is 0s. Given that the initial value for transition-property is 'all', wouldn't that mean that on a page with no specified transitions, every single property change is causing events to be thrown?
And what happens when a shorthand property is changed and a transition is kicked off? Is it a single transition with 1 event thrown for the shorthand, or are the longhands transitioning with an event thrown for each? Or are they all transitioning, with 1 event thrown for the shorthand and an event thrown for each transitioning longhand?

It'd be much appreciated if you could please clarify when a transition/animation occurs.

Thanks!
Jen