[css-color-hdr] How to account for HDR in the cascade when UAs "limit the maximum luminance at a user option."

[cookiecrook]

Accessibility section of the CSS COLOR HDR spec says:

Some individuals may have a sensitivity to very bright colors, so user agents should provide a mechanism to limit the maximum luminance at user option.

Cite: https://www.w3.org/TR/css-color-hdr-1/#a11y

Apple's Reduce White Point is an accessibility feature for users experiencing light-sensitivity; often a comorbidity of low vision.

My question is how should this (or other UA switch as recommended) be implemented to work within the cascade context of HDR images and video? I heard some proposals about adding an user agent style sheet property (or using dynamic-range-limit?) to enable HDR by default, but allow authors (or an UA/AT feature like Reduce White Point) to override the default value.

[cookiecrook]

@weinig @smfr @svgeesus FYI

[smfr]

I assume Reduce White Point acts on the entire screen? Isn't the OS-level reduction sufficient?

[cookiecrook]

Yes, and reading between the lines, you’re correct that there probably isn’t a need for a media feature. However, the spec also includes this line:

The dynamic-range-limit property could also be set to standard or constrained-high in a user stylesheet.

I presume that’s in reaction to the user setting, and it’s not clear how the two recommendations fit together.

[svgeesus]

I agree that setting it in a user stylesheet would set a useful default, but without !important would lose against a higher specificity rule in an author stylesheet.

What is needed is more of an override, like "always treat no-limit as constrained-high"