[mediaqueries-5] Add forced-colors MQ. #3807

Author: fantasai

mediaqueries-5/Overview.bs

@@ -650,6 +650,36 @@ 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 take out of context.
 
+<h3 id="forced-colors">
+Detecting a forced color palette: the 'forced-colors' feature</h3>
+
+	<pre class='descdef mq'>
+	Name: forced-colors
+	Value: none | active
+	For: @media
+	Type: discrete
+	</pre>
+
+	The 'forced-colors' media feature is used to detect if
+	the user agent has enabled a <a>forced colors mode</a>
+	where it enforces a user-chosen limited color palette on the page.
+
+	<dl dfn-type=value dfn-for="@media/prefers-color-scheme">
+		<dt><dfn>none</dfn>
+		<dd>
+			<a>Forced colors mode</a> is not active;
+			the page's colors are not being forced into a limited palette.
+
+		<dt><dfn>active</dfn>
+		<dd>
+			Indicates that <a>forced colors mode</a> is active.
+			The UA will provide the color palette to authors
+			through the CSS system color keywords
+			and, if appropriate, trigger the appropriate value of 'prefers-color-scheme'
+			so that authors can adapt the page.
+			See [[!css-color-adjust-1#forced]] for details.
+	</dl>
+
 <h2 id="changes" class="no-num">
 Changes</h2>
 <h3 id="changes-2017">