[css-color-adjust-1] More precise requirements for deriving color schemes from forced color backgrounds per #3983 (comment)

Author: fantasai

css-color-adjust-1/Overview.bs

@@ -328,18 +328,20 @@ Forced Color Schemes: the 'forced-color-adjust' property {#forced}
 	through the CSS system colors
 	(see [[CSS-COLOR-4]]).
 	Additionally,
-	if the UA determines (based on Lab lightness),
+	if the UA determines, based on Lab lightness,
 	that the canvas color
-	is clearly either dark or light
-	(for some reasonable UA delineation of “dark” or “light”),
+	is clearly either dark (L < 33%) or light (L > 67%),
 	then it must match the appropriate value
 	of the '@media/prefers-color-scheme' media query
 	and express a corresponding user preference for 'color-scheme'.
 	This will allow pages that support light/dark color schemes
 	to automatically adjust to more closely match
 	the forced color scheme.
-	Note that medium-lightness forced backgrounds may yield
-	a '@media/prefers-color-scheme' of ''prefers-color-scheme/no-preference''.
+	Behavior between the above dark vs. light thresholds
+	is UA-defined,
+	and may result in assuming either
+	''prefers-color-scheme/light'', ''prefers-color-scheme/dark'', or ''prefers-color-scheme/no-preference''
+	as the user’s preferred color scheme.
 
 <!--THOUGHTS
 	This advice (below) maybe makes sense for (prefers-contrast),