Also, I'm not understanding why HSL doesn't automatically inherit the P3 color space without any code changes. It shouldn't require any additional syntax, the HSL model is describing abstracted color values, unlike HEX or RGB which are hardcoded values.
Unless I'm not understanding this correctly. If that's correct, it seems strange and sloppy to require the display-p3 definition.
Because an HSL value translated directly to sRGB will look different when translated to P3. The color spaces themselves define different ranges of R, G, and B. The only backwards-compatible method to derive P3 colors from HSL values would be with saturation and lightness values greater than one.
HSL is not really abstracted. Instead, it is just a different way of specifying RGB values. So you can't have HSL switch to the P3 color space, because you would end up with color shifts.
In other words, same HSL => same RGB, and same RGB but different space => often a different color.
Unless I'm not understanding this correctly. If that's correct, it seems strange and sloppy to require the display-p3 definition.