HTML4 Color Keywords in HSL
The color wheel is 100% saturation throughout, and fades from a lightness of 50% around the rim to 0% in the center. The wheel and strip can be conceptually combined by rotating the grayscale strip around the X axis so that the top is pointing directly at you, and then placing the bottom of the strip on the center of the wheel. (I may do that with CSS 3D transforms at some point.)
Note that if you hover over the wheel, a “skeleton” will be revealed.