Re: [w3ctag/design-reviews] CSS Color: lab(), lch() (#488)

I think so far we've had three threads of discussion in this issue:
* discussion about alternative color spaces to `lab()` and `lch()` (entirely in this issue), for which the TAG really doesn't have the experts on, so it's probably best raised in the [w3c/csswg-drafts](https://github.com/w3c/csswg-drafts) repo (if anywhere)
* discussion (in [last week's breakout](https://github.com/w3ctag/meetings/blob/gh-pages/2020/telcons/04-20-minutes.md#css-color-lab-lch) and [plenary](https://github.com/w3ctag/meetings/blob/gh-pages/2020/telcons/04-20-minutes.md#breakout-b-1) and between them) on whether the L component of `lab()` and `lch()` would be useful for color contrast calculations:  our conclusion was that it's not, but only because it differs by a gamma correction from a value that would be useful (see the conversion from `y` to `L` in [`XYZ_to_Lab()`](https://drafts.csswg.org/css-color-4/#color-conversion-code), particularly the `Math.cbrt`).  (Although it differs from the [value currently used](https://w3c.github.io/wcag/techniques/general/G18) by both that gamma correction and by the difference between the D50 and D65 whitepoints, but the whitepoint difference seems much less significant.)
* discussion about auditing what other pieces of the web platform would need to change given the presence of colors outside of sRGB's gamut, in https://github.com/w3ctag/design-reviews/issues/488#issuecomment-610107610 and https://github.com/w3ctag/design-reviews/issues/488#issuecomment-616863174 and https://github.com/w3ctag/design-reviews/issues/488#issuecomment-618105500

We discussed this briefly in a TAG breakout today, and I think the main reason I want to leave the issue open further is to monitor progress on the third one.

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