Adds the 'prefers-contrast' feature, which lets authors adapt web content to user-selected level of contrast in the OS, such as increased contrast mode on macOS and high contrast mode on Windows. Valid options are 'more', 'less', 'forced', or 'no-preference'.
If Forced Colors Mode is enabled, 'prefers-contrast: forced' will evaluate to true, in addition to 'forced-colors: active'. Additionally, the spec mentions that "when the user agent can determine whether the forced color palette chosen by the user has a high or low contrast, one of 'prefers-contrast: more' or 'prefers-contrast: less' must match in addition to 'prefers-contrast: forced'." Using WCAG definitions[1], and in collaboration with Windows accessibility experts, we have decided to match 'more' in Forced Colors Mode if the contrast ratio between the foreground and background color is 7:1 or greater. "A contrast ratio of 3:1 is the minimum level recommended by [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] for standard text and vision"[1]. Given this, we will start by matching to 'less' in Forced Colors Mode if the contrast ratio between the foreground and background color is 2.5:1 or less. These ratios will act as an experimental baseline that we can adjust based on user feedback. [1] https://www.w3.org/WAI/WCAG21/Understanding/contrast-enhanced
The feature will be supported on all platforms, but whether the user will be able to change to a specific contrast preference may depend on the OS.