[web-animations-2] controlling animation frame rate

Apple has started working on an experimental feature (off by default in Safari Technology Preview) to control the animation frame rate of animations, with an explainer detailing an initial proposal.

To summarize, we would like to allow authors to specify a frame rate for animations such that it would be possible to identify the relative impact of animations, some preferring to run at lower frame rates – and thus allowing to optimize the cadence at which the page is rendered – and some at higher frame rates for maximum visual impact.

Authors would specify a frame rate, either via a keyword (auto, low, high, highest) or using an explicit value.

This proposal would tie in nicely with the notion of a CustomEffect (see https://github.com/w3c/csswg-drafts/issues/6861) such that scripted animations could easily adjust their frame rate without changing their logic.

Cc @smfr

Start with the linked animation frame rate explainer and compare its proposal with the related CustomEffect issue. A complete outcome would define how the auto, low, high, highest, and explicit frame-rate options should work for animations, including their interaction with scripted animations.