[mediaqueries-5] Remove (prefers-color-scheme: no-preference) per WG resolution. Add a note explaining the removal, and another explaining the preferred usage of the feature.

Author: tabatkins

mediaqueries-5/Overview.bs

@@ -619,7 +619,7 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 
 	<pre class='descdef mq'>
 	Name: prefers-color-scheme
-	Value: no-preference | light | dark
+	Value: light | dark
 	For: @media
 	Type: discrete
 	</pre>
@@ -628,24 +628,27 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 	desire that the page use a light or dark color theme.
 
 	<dl dfn-type=value dfn-for="@media/prefers-color-scheme">
-		<dt><dfn>no-preference</dfn>
-		<dd>
-			Indicates that the user has made no preference known
-			to the system. This keyword value evaluates as false
-			in the <a>boolean context</a>.
-
 		<dt><dfn>light</dfn>
 		<dd>
 			Indicates that user has expressed the preference for
-			a page that has a light theme (dark text on light background).
+			a page that has a light theme (dark text on light background),
+			or has not expressed an active preference
+			(and thus should receive the "web default" of a light theme).
 
 		<dt><dfn>dark</dfn>
 		<dd>
 			Indicates that user has expressed the preference for
 			a page that has a dark theme (light text on dark background).
-
 	</dl>
 
+	Note: The values for this feature might be expanded in the future
+	(to express a more active preference for light color schemes,
+	or preferences for other types of color schemes like "sepia").
+	As such, the most future-friendly way to use this media feature
+	is by directly testing against the values you wish to match,
+	such as ''(prefers-color-scheme: light)'' and ''(prefers-color-scheme: dark)'',
+	rather than by negation such as ''(prefers-color-scheme: light)'' and ''(not (prefers-color-scheme: light))''.
+
 	The method by which the user expresses their preference can vary.
 	It might be a system-wide setting exposed by the Operating System,
 	or a setting controlled by the User Agent.
@@ -660,6 +663,19 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 	so that 'prefers-color-scheme' reflects preferences appropriate to the medium
 	rather than preferences taken out of context.
 
+	<div class=note>
+		This feature, like the other 'prefers-*' features,
+		previously had a <css>no-preference</css> value
+		to indicate an author not expressing an active preference.
+		However, user agents converged on expressing the "default" behavior
+		as a ''light'' preference,
+		and never matching <css>no-preference</css>.
+
+		If a future user agent wishes to expose a difference
+		between "no preference" and "really wants a light display",
+		please contact the CSSWG to discuss this.
+	</div>
+
 <h3 id="forced-colors">
 Detecting a forced color palette: the 'forced-colors' feature</h3>