[Webkit-unassigned] [Bug 212409] New: [Web Animations] CSS-originated and JS-originated keyframe animations with implicit values behave differently

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=212409

            Bug ID: 212409
           Summary: [Web Animations] CSS-originated and JS-originated
                    keyframe animations with implicit values behave
                    differently
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: graouts at webkit.org
                CC: dino at apple.com

Consider this example:

    const keyframes = { left: "500px" };
    document.querySelector(".single").animate(keyframes, 1000);
    document.querySelector(".duplicate").animate(keyframes, 1000);
    document.querySelector(".duplicate").animate(keyframes, 1000);

In this example, the .single and .duplicate elements will not have the same "left" value during the span of the animation as the second animation for .duplicate will use the "left" value animated by the first animation as the implicit "from" value.

However, the same example written using a CSS Animation will behave differently and .single and .duplicate will have the same animated value for each frame. I haven't checked, but I think this is due to our resolving values for implicit keyframes for CSS Animations when they start rather than at run time.

I was a little baffled though by the fact that both Firefox and Chrome will return a static "from" value when calling getKeyframes() on the individual animations. That doesn't seem to make sense, I'd expect the "left" value ought to be left out or computed to be what is current.

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