Comment by refulgentis

c.f. https://news.ycombinator.com/item?id=44015980, when you cut out the incorrect stuff due to confusion re: APCA's button example, it's a bit clearer that it's 100% right.

Consistent, it is not. Ex. we can imagine a background at L* 50 that is ~equally served with a white or black foreground - in that case, the aesthetic principles come into play.

To also disambiguate that, and get to 100% reliable, if both a darker and lighter color are available given contrast K and background color C, look at C, if it's L* is >= 60, choose lighter.

Then, it is 100% correct and consistent.