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Re: [csswg-drafts] [css-animations] Resolving dependencies in keyframes (#5125)

From: andruud via GitHub <sysbot+gh@w3.org>
Date: Wed, 10 Jun 2020 10:28:08 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-641911017-1591784887-sysbot+gh@w3.org>
> The object returned represents the actual values we are interpolating between at that point.

I admit I didn't think about it that way. In isolation that's fair enough.

> Interpolation happens in computed value space so we have to convert to computed values to interpolate and the object returned accurately represents that.

Didn't think about it that way either. I was thinking interpolation happens in specified value space, e.g. interpolation from `10em` to `20em` becomes `15em` at t=0.5, and then the `em` is resolved. Sounds like your mental model is that both `10em` and `20em` endpoints are dynamically resolved, and then interpolation happens on those values in `px`. I guess there's no observable difference.

> doesn't make these values "random" any more than the values from getComputedStyle are "random"

If you do `div { font-size: 10px; width: 10em; }`, and then round-trip the `getComputedStyle` result of `width`, the width will no longer be responsive to changes in `font-size`, but you'll at least get the same computed value for `width` beyond that.

And if you do:

@keyframes test {
  from { font-size: 10px; width: 10em; }
  to { font-size: 20px; width: 20em; }

And roundtrip that with `getKeyframes()/setKeyframes()` at t=0 for example, you get an animation for `width` that's no longer responsive to changes in `font-size`, so maybe that's indeed consistent with gCS. But you also get an interpolation for `width` that's different than the one you would otherwise get. On the one hand this is natural consequence of the interpolation not being responsive to `font-size` any more, but on the other hand it feels unnecessarily "unstable". If gCS is able to roundtrip a specific _value_, it seems to fair to at least initially expect that get/setKeyframes should be able to roundtrip a specific _interpolation_, and not just a specific state of the interpolation.

> getComputedKeyframes()

Is that basically what this is? :slightly_smiling_face:

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