[css-color-hdr] No HDR color-difference (ΔE) metric specified

[svgeesus]

CSS Color HDR defines `IC_TC_P color space for use in CSS, and the description notes:

Also unlike Lab, which has primarily been tested with lower-intensity reflective colors, ICTCP has been tested with high-chroma, self-luminous, and high intensity (HDR) colors. This makes it suitable for color difference measurement (deltaE ITP) and for gamut mapping of SDR and HDR colors.

However, deltaE ITP is not defined or referenced in that spec.

CSS Color 4 references both ΔE 2000 and ΔEOK, including sample code, but these are both SDR color difference metrics.

For computing the perceptual color difference of two HDR colors (or one HDR and one SDR) ΔE ITP should be recommended and there should be sample code for ease of implementation and to compare different implementations.