[css-values-4] Out of range progress of generic interpolation function / mix()

[fantasai]

In #581 and #2854 we resolved to add a generic interpolation function, which takes a percentage, a start value, and an end value.

  mix( [ <percentage> && [ by <easing-function> ]? ] ; <start-value> ; <end-value>)

One outstanding question is, what happens if the progress percentage is outside the 0-100% range? Do we clamp the values to the range? Do we interpolate past the range? Do we need a syntactic switch between the two (e.g. a clamp keyword in the first arg)?

[tabatkins]

Out-of-range interpolation is already part of how CSS's interpolation mechanics work (you need it to handle some cubic-bezier(), if nothing else); we don't need to do anything special for this.

[Loirooriol]

The percentage represents the input progress value, right? That's in the range [-∞, ∞]. So I wouldn't clamp. If some authors want to clamp, they can use clamp(0%, var(--progress), 100%) or something.