[compositing] Incorrect recommended implemntation of mix-blend-mode: saturation

RE: https://www.w3.org/TR/compositing-1/#valdef-blend-mode-saturation

The math used in the functions:

Lum(C) = 0.3 x Cred + 0.59 x Cgreen + 0.11 x Cblue

and

Sat(C) = max(Cred, Cgreen, Cblue) - min(Cred, Cgreen, Cblue)

seems to be taken from the calculations for Luma and Chroma (respectively)
in the LCH color space (as seen here
<https://en.wikipedia.org/wiki/HSL_and_HSV#Hue_and_chroma> and here
<https://en.wikipedia.org/wiki/HSL_and_HSV#Luma,_chroma_and_hue_to_RGB>).
However, the expectation would be that it's derived from
lightness/luminance and saturation from the HSL or HSV/HSB color spaces.
This expectation stems from the fact that the word chosen to describe the
blend mode is "saturation", not "chroma" or "chrominance". This has lead to
entirely too much confusion with people thinking there's a bug in the
implementation of mix-blend-mode: saturation.

My plea is that either the saturation blend-mode gets renamed to "chroma"
or "chrominance" or the implementation gets fixed to match a colorspace
that actually has saturation as one of its channels (ideally HSL as that
colorspace exists in the svg/css world already).

Cheers,
Callum Morrisson