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

One outstanding question is: where is this notation valid?

• It definitely needs to be valid as a top-level function that wraps the entire property's value.
• Can it also be used as a lower-level syntax, e.g. for interpolating <length> values? What about <position> values (which have multiple components)?