[web-animations-1] Allow keyframe offsets outside the range [0,1]?

[birtles]

See issue at the end of: https://drafts.csswg.org/web-animations-1/#the-effect-value-of-a-keyframe-animation-effect

[Nadya678]

cubic-bezier(0, -0.5, 1, 1.5)?
Current in use for colours by me. It works in all modern browsers in the same way.

[birtles]

This is about being able to specify keyframes at, e.g. -10% so that when you have a timing function of cubic-bezier(0, -0.5, 1, 1.5) you can control the interpolation (or actually extrapolation) outside the 0%~100% range.

[Nadya678]

it is natural: only 0% to 100% (from is 0% and to is 100%). Before "from" and after "to"? It will be unintuitive. the -10% and 110% shall be parsed as errors.