The examples on the page are all at the same lightness, so you only see a slice of possible colors. The main point it's trying to show is that HSL's notion of l

The examples on the page are all at the same lightness, so you only see a slice of possible colors. The main point it's trying to show is that HSL's notion of lightness is rather useless. When it's trying to generate some background colors (with a fixed lightness of 60%), note that some of the HSL colors are much brighter than others, to the point that the white text becomes hard to read. By contrast, all of the HUSL colors at 60% are suitable for use as a background under white text. You can press the test buttons again to see more examples.