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)?