H (hue) is an angle of the color circle given in degs, rads, grads, or turns. When written as a unitless number, it is interpreted as degrees. Red=0deg=360deg, green=120deg, blue=240deg, etc. It implicitly wraps around such that -120deg=240deg, 480deg=120deg, -1turn=1turn.

S (saturation) and L (lightness) are percentages. 100% saturation is completely saturated, while 0% is completely unsaturated (gray). 100% lightness is white, 0% lightness is black, and 50% lightness is “normal.”

A (alpha) can be a number between 0 and 1, or a percentage, where the number 1 corresponds to 100% (full opacity).