Re: [csswg-drafts] [css-color][css-animations] Is it specified anywhere that a color animation at time=0 exactly match the from color's computed value? (#13269)

> Is this behavior, of acting like no interpolation happed at all, specified anywhere?
Yes, it falls out of the [interpolation procedure](https://drafts.csswg.org/css-values-4/#interpolation) which has:

>> Given two property values VA and VB, produces an intermediate value Vresult at a distance of p along the interval between VA and VB such that p = 0 produces VA and p = 1 produces VB.

@birtles Thanks! Ok, I can totally see how that would mean that at time=0, and time=1 should return the identical serialization, since it calls out "p = 0 produces VA and p = 1 produces VB". Great!

What about at other explicit stops. For example, what would you expect the computed value serialization be for the following:

```css
  @keyframes bg-color-legacy-rgb {
    from {
      background-color: red;
    }
    50% {
      background-color: lime;
    }
    to {
      background-color: blue;
    }
  }
```

Not sure where in the spec the interpolation procedure is invoked, so I can't tell if the interval (p=0 to p=1) is exactly time=0, to time=1, or if it is percentage of the current keyframe (I kinda assume animations would invoke it per-keyframe).

So (and I will make a new WPT test with these answers), what should the computed value serialization be for that animation above at various timesteps:

I think I know these first three:

- At time=0, we are saying the computed style serialization would be `rgb(255, 0, 0)`.
- At time=1, we are saying the computed style serialization would be `rgb(0, 0, 255)`.
- At time=0.25 (or any non-explicit stop), we are saying the computed style serialization would be some `oklab()` value.

But what about time=0.5? Should it be `rgb(0, 255, 0)` or `oklab(87% -0.23 0.18)`?

-- 
GitHub Notification of comment by weinig
Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13269#issuecomment-3695001251 using your GitHub account


-- 
Sent via github-notify-ml as configured in https://github.com/w3c/github-notify-ml-config

Received on Sunday, 28 December 2025 19:56:18 UTC