HUSL is a rather interesting colorspace. It's like a more sRGB-friendly version of Lch, intended as a fairly direct upgrade on HSV/HSL/HCY. It maps the part of LCH space that contains the sRGB colorspace to/from normalized H/'S'/'L' coordinates, so that all valid coordinates in HSL space are valid colors in sRGB. I quote 'S' and 'L' because these are not the same S and L used in HSL.
Seems to be quite useful for calculating color ramps. The page includes Javascript, Python, and Lua implementations of RGB <-> HUSL conversion, as well as an interactive color selector.
I am a little puzzled as to why they are using Lch(uv) as a basis when Lch(ab) is generally more consistent. Still, it's rather clearly a step up from HSL.
A snip from the page, which gives a simplified comparison of HSL/LCH/HUSL characteristics:
