[css-color-adjust-1] Per WG resolution, restrict 'only' to 'light' for now, with explanatory issue.

Author: tabatkins

css-color-adjust-1/Overview.bs

@@ -80,7 +80,7 @@ Opting Into a Preferred Color Scheme: the 'color-scheme' property {#color-scheme
 
 	<pre class=propdef>
 	Name: color-scheme
-	Value: auto | only? && [ light | dark | <<custom-ident>> ]+
+	Value: auto | [ light | dark | <<custom-ident>> ]+ | only && light
 	Initial: auto
 	Applies to: all elements
 	Inherited: yes
@@ -130,13 +130,20 @@ Opting Into a Preferred Color Scheme: the 'color-scheme' property {#color-scheme
 			is valid but has no additional effect
 			beyond what the first instance of the keyword provides.
 
-		: <dfn lt=only>only?</dfn>
+		: <dfn lt=only>only</dfn>
 		:: If provided,
 			''only'' indicates that the element <em>must</em> be rendered with one of the specified color schemes,
 			if possible,
 			even if the user's preference is for a different color scheme.
 
-			Authors <em>should not</em> use this value,
+			Issue: Per spec, ''only'' can only be used with ''light''.
+			Apple's implementation allows it with ''dark'' as well.
+			The concern with ''only dark'' is that pages won't consider UAs that can't support ''dark'' schemes,
+			and will thus render brokenly.
+			This value might be expanded to all schemes or removed entirely
+			depending on further consideration.
+
+			Authors <strong>should not</strong> use this value,
 			and should instead ensure that their page renders well with whatever color scheme the user prefers
 			(using the '@media/prefers-color-scheme' media query to adjust styles accordingly).
 			This keyword is provided only for the rare cases where that might not be reasonably possible,