[css-color-hdr] Absolute luminance of PQ #10460
Description
The spec makes the follow claim (https://drafts.csswg.org/css-color-hdr/#valdef-color-rec2100-pq):
In contrast to SDR color spaces and some HDR color spaces, the PQ values are absolute rather than relative.
I think this is at best very confusing. For example, rec2100-linear has the exact same peak white luminance, black luminance and white luminance but isn't said to have "absolute values". Are the values behaving differently? I don't believe they should.
To take it even further, sRGB according to the "Controlling Dynamic Range" section (https://drafts.csswg.org/css-color-hdr/#controlling-dynamic-range) has (reference and peak) white defined at a luminance of 80 cd/m² and black at 0.2 cd/m². Is this not absolute? How is this different than the PQ absoluteness?
Personally, I don't think that PQ is any more or less absolute than any other fully color space + reference display + reference viewing environment combination, it just chose put the absolute luminance directly in the definition of the curve instead of implicitly over the definition of the reference display.
This has been abused to not properly anchor SDR and HDR content and, at the actual display, have HDR content at a fixed luminance, while SDR content will have different luminances depending on some brightness setting.