Refactoring of the forced-colors and prefers-contrast media feature (w3c#5434)

This editorial reorganization of the text is intended to make the
overlap between forced-colors:active and prefers-contrast:forced
more readily understandable: one being the syntax that the CSS Working
Group finds more desirable, the other being retained for compatibility
with content issued against an earlier draft (see w3c#3856).

Prior to this refactoring, the definition of this mode was also spread
between the two media features, causing confusion. This commits
consolidates the whole thing in one place.

Related to w3c#5433

Author: frivoal

mediaqueries-5/Overview.bs

@@ -2624,7 +2624,7 @@ Detecting the desire for reduced transparency on the page: the 'prefers-reduced-
 	ISSUE: How does this interact with preferences around e.g. pattern fills and backgrounds?
 	They're not about transparency, but they also interfere with shape recognition.
 
-<h3 id="prefers-contrast">
+<h3 id="prefers-contrast" oldids="forced-colors">
 Detecting the desire for increased or decreased color contrast from elements on the page: the 'prefers-contrast' feature</h3>
 
 	<pre class='descdef mq'>
@@ -2634,11 +2634,12 @@ Detecting the desire for increased or decreased color contrast from elements on
 	Type: discrete
 	</pre>
 
-	The 'prefers-contrast' media feature is used to detect if the user
-	has requested the system increase or decrease the amount of contrast
-	between adjacent colors. For example, many users have difficulty
-	reading text that has a small difference in contrast to the text
-	background and would prefer a larger contrast.
+	The 'prefers-contrast' media feature is used to detect
+	if the user has requested more or less contrast in the page.
+	This could be responded to, for example,
+	by adjusting the contrast ratio between adjacent colors,
+	or by changing how much elements stand out visually,
+	such as by adjusting their borders.
 
 	<dl dfn-type=value dfn-for="@media/prefers-contrast">
 		<dt><dfn>no-preference</dfn>
@@ -2659,10 +2660,12 @@ Detecting the desire for increased or decreased color contrast from elements on
 
 		<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]].
+			Indicates that <a>forced colors mode</a> is active:
+			the user agent enforces a user-chosen limited color palette on the page,
+			in effect also implying a strong preference for the associated contrast.
+			The UA will provide the color palette to authors
+			through the CSS system color keywords.
+			See [[css-color-adjust-1#forced]] for details.
 
 			<div class=advisement>
 				This does <em>not</em> necessarily indicates
@@ -2673,10 +2676,18 @@ Detecting the desire for increased or decreased color contrast from elements on
 				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: more'' or ''prefers-contrast: less'' should match,
+			Additionally, when the user agent can determine
+			whether the forced color palette chosen by the user
+			has a high or low contrast,
+			one of ''prefers-contrast: more'' or ''prefers-contrast: less'' must match
 			in addition to ''prefers-contrast: forced''.
+
+			Note:
+			Similarly,
+			if the forced color palette chosen by the user
+			fits within one of the color schemes described by 'prefers-color-scheme',
+			the corresponding value will also match.
+			See [[css-color-adjust-1#forced]] for details.
 	</dl>
 
 	<div class=note>
@@ -2702,6 +2713,51 @@ Detecting the desire for increased or decreased color contrast from elements on
 		result in a high or low contrast (or something else).
 	</div>
 
+	<div class=example>
+		Preference for more or less contrast may arise
+		from a variety of different situations.
+		Here are some examples:
+
+		<ul>
+			<li>
+				Many users have difficulty
+				reading text that has a small difference in contrast to the text
+				background and would prefer a larger contrast.
+
+			<li>
+				People suffering from migraine
+				may find strongly contrasting pages
+				to be visually painful
+				and would prefer a low contrast.
+
+			<li>
+				Some people with dyslexia find high contrast text hard to read,
+				as they feel that the letters shine / sparkle as if backlit by too bright a light,
+				and find low contrast to be more comfortable.
+
+			<li>
+				Environmental factors may also lead
+				a user to prefer more or less contrast.
+				See also [[#auto-pref]].
+		</ul>
+
+		Issue: This list should be refined and expanded.
+	</div>
+
+	For compatibility with legacy content,
+	the following syntax is also supported:
+
+	<pre class='descdef mq'>
+	Name: forced-colors
+	Value: none | active
+	For: @media
+	Type: discrete
+	</pre>
+
+	<dfn type=value dfn-for="@media/prefers-color-scheme" export>active</dfn> matches
+	when ''prefers-contrast: forced'' matches,
+	otherwise <dfn type=value dfn-for="@media/prefers-color-scheme" export>none</dfn> matches.
+
 <h3 id="prefers-color-scheme">
 Detecting the desire for light or dark color schemes: the 'prefers-color-scheme' feature</h3>
 
@@ -2763,36 +2819,6 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 		please contact the CSSWG to discuss this.
 	</div>
 
-<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>
-
 <h3 id="prefers-reduced-data">
 Detecting the desire for reduced data usage when loading a page: the 'prefers-reduced-data' feature</h3>
 
@@ -3212,4 +3238,3 @@ Comments from
  akklesed,
  and 張俊芝
 improved this specification.
-