https://drafts.csswg.org/css-color-hdr-1/#hdr-color-function

The two headroom values must not be the same

1. What should happen if the two headroom values are the same? Should the function be invalid?

2. In color-hdr(0 xyz-d65 1.2 0 0, xyz-d65 1.2 0 0), are they the same value? Should the default value be 0 when omitted, as is suggesting the CSSWG discussion that led to the resolution to add color-hdr()?

3. I think its <color> components should serialize as resolved in [css-color-5] Should colors nested in a parent color function (RCS, color-mix, light-dark, color-contrast) serialize in their most precision saving form #10328 but since the corresponding spec edits are still pending, could it be clarified in this issue?

I suspect that hdr-color() = color-hdr(...) is intentional. But that seems confusing, imo.