[css-color-4] Tweak intro to deprecated system colors.

Author: tabatkins

css-color-4/Overview.bs

@@ -2867,18 +2867,23 @@ Default Style Rules</h2>
 <h2 id="deprecated-system-colors" class="no-num">
 Appendix A: Deprecated CSS System Colors</h2>
 
-	Earlier versions of CSS defined several additional <<system-color>>s.
-	These color names have been <strong>deprecated</strong>, however,
+	Earlier versions of CSS defined several additional [=system colors=].
+	These color keywords have been <strong>deprecated</strong>, however,
 	as they are insufficient for their original purpose
 	(making website elements look like their native OS counterparts),
-	and represent a security risk,
-	as it makes it easier for a webpage to "spoof" a native OS dialog.
+	represent a security risk
+	by making it easier for a webpage to “spoof” a native OS dialog,
+	and increase fingerprinting surface, compromising user privacy.
 
 	User agents must support these keywords,
-	but should map them to "default" values,
+	but are encouraged to map them to default values
 	not based on the user's OS settings
-	(for example, mapping all the "background" colors to white and "foreground" colors to black).
-	Authors must not use these keywords.
+	or match them to appropriate (undeprecated) [=system colors=].
+	<strong>Authors must not use these keywords.</strong>
+
+	The deprecated system colors are represented
+	as the <dfn><<deprecated-colors>></dfn> sub-type,
+	and are defined as:
 
 	<dl noexport>
 		<dt><dfn>ActiveBorder</dfn>