[css-color-6] Does color-contrast() need to support three color contrast comparisons?

[CharlesBelov]

• please be specific (in the title and issue) about what you want to change:

So far as I can tell, color-contrast() intends to support foreground versus background. However, for sites which hide link underlining or other link marking, the site would also need to support foreground versus link and background versus link. For example, foreground versus link for WCAG 2.0 AA needs to have 3:1 contrast and background versus link for WCAG 2.0 AA needs to have 4.5:1 contrast.

So, does color-contrast also need a syntax and means to support the three-way comparison of:

• foreground versus background
• foreground versus link
• background versus link
  ?

• please link to the spec section you're talking about, or at least the spec
  2. Computing a Contrasting Color

[danburzo]

I think there are two aspects worth discussing here that may influence the final syntax for contrast-color() (#7937):

• Is there a need for an additional contrast keyword, or a different set of keywords altogether, to express the relationship between "adjacent but not overlapping" colors? ([css-color-6] color-contrast() should distinguish foreground and background #7359)
• Is there a need for the function to support multiple constraints for the set of candidate colors? What happens when you pit the default color candidates (white, black) against possibly-conflicting constraints?

Your link color vs. text and background could possibly be expressed along the lines of:

a {
  color: contrast-color(adjacent currentColor wcag2(3), background var(--bg) wcag2(4.5), blue, violet, etc.)
}

Support for multiple constraints will most likely impose some limitations on the syntax in order to disambiguate constraints from candidate colors.