[css-anchor-position] Simplifying math functions for `<percentage>` in `anchor()`?

[dshin-moz]

anchor() allows specifying percentage values, which interpolates between start and end.

This seems like something that can be simplified, since I think the basis of that will always be positive, since it depends on the anchor element's size in that axis. OTOH, WPT test anchor-parse-valid.html expect the min(50%, 100%) value to round trip.