Re: [csswg-drafts] [css-transitions-1] Clarify timing of transitionrun and transitionstart (#11665)

> > Internally Firefox has an animation observer interface which is used by DevTools.
> 
> Do you have more details about this animation observer? I’m curious about it :)

The basic interface is [here](https://searchfox.org/firefox-main/source/dom/base/nsIAnimationObserver.h) and you can see some of the implementation [here](https://searchfox.org/firefox-main/rev/b30a9b734819436853abf4eba6d768037514b3f6/dom/base/nsDOMMutationObserver.cpp#347) for example.

> Also: wouldn’t this animation observer be affected by this same timing incompatibility? I mean, if it were ever to became a standardized thing, then it would also need the fix detailed in the OP, no?

I think the expected behavior is that if a style flush is triggered, we should already be firing `transitionrun` before the next requestAnimationFrame (which happens to be the current Firefox behavior).

From a spec point of view, I think the definition in [level 2](https://drafts.csswg.org/css-transitions-2/#transition-events) is pretty clear but could benefit from a s/idle/idle or non-existent/.

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Received on Friday, 10 October 2025 00:12:53 UTC