[mediaqueries-5] Add "prefers-contrast:forced"

Closes w3c#3856

Author: frivoal

mediaqueries-5/Overview.bs

@@ -2714,7 +2714,7 @@ Detecting the desire for increased or decreased color contrast from elements on
 
 	<pre class='descdef mq'>
 	Name: prefers-contrast
-	Value: no-preference | high | low
+	Value: no-preference | high | low | forced
 	For: @media
 	Type: discrete
 	</pre>
@@ -2742,8 +2742,51 @@ Detecting the desire for increased or decreased color contrast from elements on
 			Indicates that user has notified the system that they
 			prefer an interface that has a lower level of contrast.
 
+		<dt><dfn>forced</dfn>
+		<dd>
+			Indicates that <a>forced colors mode</a> is active,
+			identically to ''forced-colors: active''.
+			See [[#forced-colors]] and
+			[[css-color-adjust-1#forced]].
+
+			<div class=advisement>
+				This does <em>not</em> necessarily indicates
+				a preference for a high contrast.
+				The colors have been forcibly adjusted
+				to match the preference of the user,
+				but that preference can be for a low contrast or a high one,
+				or some other arrangement that is neither particularly low or high contrast.
+			</div>
+
+			Note: When the User Agent can determine whether the particular forced color scheme
+			chosen by the user is high or low contrast,
+			one of ''prefers-contrast: high'' or ''prefers-contrast: low'' should match,
+			in addition to ''prefers-contrast: forced''.
 	</dl>
 
+	<div class=note>
+		Note: Authors can respond to specific user preferences for a higher or lower
+		contrast using ''prefers-contrast: high'' or ''prefers-contrast: low'',
+		as appropriate.
+
+		Using an unqualified <code class=lang-css>@media (prefers-contrast) { … }</code>
+		to apply high contrast styles is incorrect and user-hostile,
+		as it would also impose high contrast styles to people who have requested the exact opposite.
+
+		However, it is also common to reduce visual clutter
+		in response to both high and low contrast preferences.
+		In that case, it is appropriate to use <code class=lang-css>@media (prefers-contrast) { … }</code>
+		without specifying ''prefers-contrast/high'' or ''prefers-contrast/low'',
+		to do things like replacing background images with plain colors,
+		turning off decorative gradients,
+		or replacing border images or box shadows with simple solid borders.
+		As ''prefers-contrast/forced'', like ''prefers-contrast/high'' or ''prefers-contrast/low'',
+		evaluates to true in a [=boolean context=],
+		this syntax also benefits users of <a>forced colors mode</a>,
+		even when it isn't know whether their colors of choice
+		result in a high or low contrast (or something else).
+	</div>
+
 	ISSUE(2943): Split ''prefers-contrast/high'' into two levels,
 	“extremely high” (as used in MSFT's black-on-white high contrast theme)
 	and “increased (as implemented in Apple's Increased Contrast settings)?
@@ -2836,7 +2879,7 @@ Detecting a forced color palette: the 'forced-colors' feature</h3>
 			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.
+			See [[css-color-adjust-1#forced]] for details.
 	</dl>
 
 <h3 id="prefers-reduced-data">