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Test for support of changing Custom Properties in Keyframes (in the context of a single div gradient piano). Seems to work in latest Chrome (as of May 2017), but not Firefox or Safari. Alternative is to use requestAnimationFrame in JS. Finally, shares basic example for how to provide interaction even in a single div. Impractical because you lose the benefits of cancel/move/leave etc for individual elements, but could technically be built out fully.
This Pen uses: HTML, CSS, JavaScript, and