Re: [w3ctag/design-reviews] Wide gamut support for Canvas/OffscreenCanvas/ImageBitmap (#315)

> We are not releasing the full WICG proposal, only the wide-gamut part of it. When in wide-gamut the color space defaults to extended sRGB. 

The term `extended sRGB` is not defined, or even used, in the explainer. As @dbaron correctly notes, the sRGB transfer function is undefined outside [0, 1] and there are multiple plausible ways of extending it, so at a minimum that needs to be fully defined. Having done so, the capability of the extended space to represent WGB (and indeed, HDR) can be evaluated.

The explainer merely notes that 8bit " is below the capabilities of some monitors" which is true, but fails to connect that statement to *why* 10 or 12 bits is needed:

- to represent wider gamuts (or wider dynamic range) without increasing the size of steps between adjacent color values (which would give banding, etc)
- to allow correction of tone curves for calibration and linearization (this is why some monitors have, for example, 14bit lookup tables for gamma correction, even when the inputs are 8,10 or 12bit)

@dbaron makes the good point that Rec.2020 uses 12bits (in fact either 10 or 12, UHD uses 10 with 2020) while the proposed 16-bit half-float has 11 effective bits. That might well  be okay, but it would be good to know that half-float is not painting us into a corner.

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