[color] Swap the meaning of percentages for the contrast adjuster. It's clear that 'contrast 100%' should mean the maximum contrast.
[color] Make the <percentage> optional in contrast.
[color] Switch <color-adjuster> over to a functional syntax.  It's pretty clear that it's more readable that way, after playing with it for a bit.
[color] Fix error - null hue angle should make s() fail when it adjusts the channel to greater than 0%.