Summary: The animation-composition property defines the composite operation used when multiple animations affect the same property simultaneously. We support the similar feature in Web Animations (for script-generated animations) already, and this new CSS property is for CSS Animations. The values of animation-composition have the meaning defined for the
corresponding values of the composite operation defined in Web
Animations. There is an example in the spec link if you are interested.
Platform coverage: all
DevTools bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1778407
1) Blink: doesn't support it.
2) Webkit: supports this already since January, 2022. (https://bugs.webkit.org/show_bug.cgi?id=232086) but has not shipped yet.
web-platform-tests: The wpt has been merged since https://bugzilla.mozilla.org/show_bug.cgi?id=1748435, in css/css-animations/ folder.