[css-color-4] browser-generated pairs should AA contrast

Author: svgeesus

css-color-4/Overview.bs

@@ -959,6 +959,12 @@ System Colors</h3>
 	to ensure legibility and consistency across the page
 	and avoid an uncoordinated mishmash of user-forced and page-chosen colors.
 
+	When the values of <<system-color>> keywords come from the browser,
+	(as opposed to being OS defaults or user choices) the browser should
+	ensure that <a href="#system-color-pairs">matching
+	foreground/background pairs</a> have a minimum
+	of WCAG AA contrast.
+
 	Authors <em>may</em> also use these keywords at any time,
 	but <em>should</em> be careful to use the colors
 	in <a href="#system-color-pairs">matching background-foreground pairs</a>