[css-color-4] system colors have meaning outside forced-colors, reflect light and dark mode #4883

Author: svgeesus

css-color-4/Overview.bs

@@ -925,7 +925,29 @@ Note: The history of the X11 color system is interesting,
 <h3 id="css-system-colors" dfn lt="system colors">
 System Colors</h3>
 
-	In [=forced colors mode=],
+	In general, the <<system-color>> keywords
+	reflect <em>default</em> color choices made by the user, the browser, or the OS.
+	They are typically used in the brwser default stylesheet, for this reason.
+
+	To maintain legibility,
+	the <<system-color>> keywords also respond to light mode or dark mode changes.
+
+
+	<div class="example" id="LM-DM-links">
+		For example, traditional blue link text is legible on a white background
+	(WCAG contrast 8.59:1, AAA pass)
+	but would not be legible on a black background
+	(WCAG contrast 2.44:1, AA fail).
+	Instead, a lighter blue such as #81D9FE would be used in dark mode
+	(WCAG contrast 13.28:1, AAA pass).
+		<div style="margin:1em; margin-left: 2em; text-decoration: underline; background: inherit">
+		<p style="padding: 4px; background-color: white; color: blue"> Legible link text</p>
+		<p style="padding: 4px; background-color: black; color: blue"> Illegible link text</p>
+		<p style="padding: 4px; background-color: black; color: #81D9FE"> Legible link text</p>
+		</div>
+	</div>
+
+	However, in [=forced colors mode=],
 	most colors on the page are forced into a restricted, user-chosen palette.
 	The <dfn>&lt;system-color></dfn> keywords
 	expose these user-chosen colors