[css-ui-4] Define behavior for (semi) transparent accent colors

See #9852

css-ui-4/Overview.bs

@@ -2098,6 +2098,13 @@ Widget Accent Colors: the 'accent-color' property</h3>
 		</table>
 	</div>
 
+	If the color supplied is partially or fully transparent,
+	the user agent must first pre-compose it over the ''<system-color>/Canvas'' system color
+	and use the resulting (now opaque) color as the accent color.
+
+	Note: Although it may vary based on user settings,
+	''<system-color>/Canvas'' is typically white when in a [=light color scheme =]
+	and black when in a [=dark color scheme=].
 
 <h3 id="appearance-switching" caniuse="css-appearance">
 Switching appearance: the 'appearance' property</h3>