Re: [csswg-drafts] [css-transitions] starting an animation and a transition with equal values yields some incompatible behavior between Safari, Chrome and Firefox (#8701)

Debugging WebKit, I can explain the Safari behavior. For each frame, we fall into the following case from the [Starting of transitions](https://w3c.github.io/csswg-drafts/css-transitions-1/#starting) section of the CSS specification:

> If the element has a [running transition](https://w3c.github.io/csswg-drafts/css-transitions-1/#running-transition) for the property, there is a [matching transition-property value](https://w3c.github.io/csswg-drafts/css-transitions-1/#matching-transition-property-value), and the [end value](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-end-value) of the running transition is not equal to the value of the property in the [after-change style](https://w3c.github.io/csswg-drafts/css-transitions-1/#after-change-style), then:

… and then:

> Otherwise, implementations must cancel the [running transition](https://w3c.github.io/csswg-drafts/css-transitions-1/#running-transition) and start a new transition whose:
> - [start time](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-start-time) is the time of the [style change event](https://w3c.github.io/csswg-drafts/css-transitions-1/#style-change-event) plus the [matching transition delay](https://w3c.github.io/csswg-drafts/css-transitions-1/#matching-transition-delay),
> - [end time](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-end-time) is the [start time](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-start-time) plus the [matching transition duration](https://w3c.github.io/csswg-drafts/css-transitions-1/#matching-transition-duration),
> - [start value](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-start-value) is the [current value](https://w3c.github.io/csswg-drafts/css-transitions-1/#current-value) of the property in the [running transition](https://w3c.github.io/csswg-drafts/css-transitions-1/#running-transition),
> - [end value](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-end-value) is the value of the property in the - > - [after-change style](https://w3c.github.io/csswg-drafts/css-transitions-1/#after-change-style),
> - [reversing-adjusted start value](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-reversing-adjusted-start-value) is the same as the [start value](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-start-value), and
> - [reversing shortening factor](https://w3c.github.io/csswg-drafts/css-transitions-1/#transition-reversing-shortening-factor) is 1.

It's not clear to me (yet) if that is correct.

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Received on Tuesday, 11 April 2023 14:48:31 UTC