Unlike CSS Color 4, the color spaces in CSS Color HDR do not list reference ranges. This is particularly important for any polar spaces, to establish the ε for achromatic hues which depends on the extent of the reference range.

• lab(50% 125 125) = color(ictcp 0.4048 -0.0495 0.4906) = color(jzczhz 0.1355 0.2217 33.3)
• lab(50% -125 125) = color(ictcp 0.4032 -0.4135 -0.1191) = color(jzczhz 0.0895 0.2061 147.3)
• lab(50% 125 -125) = color(ictcp 0.4476 0.3446 0.0327) = color(jzczhz 0.1535 0.1947 291.1)
• lab(50% -125 -125) = color(ictcp 0.4263 0.1046 -0.4513) = color(jzczhz 0.1067 0.4211 212.1)

Suggests that for ICtCp that Ct and Cp could range from say -0.4 to +0.4 and for JzCzhz that Cz could range from 0 to 0.26 ?

Needs a more complete examination (the P3 primaries and secondaries all converted).