[css-fonts-4] May ignore turning required features off. Fix w3c#7634

Author: svgeesus

css-fonts-4/Overview.bs

@@ -6124,6 +6124,15 @@ will depend upon the way the font is designed
 (specifically, which type of lookup is used
 to define the feature).</p>
 
+<p>Implementations may choose to ignore
+turning off features which
+the OpenType specification says are always required,
+such as required ligatures "rlig".</p>
+
+<wpt>
+	font-variant-ligatures-11.optional.html
+</wpt>
+
 <p>Although specifically defined for OpenType feature tags,
 feature tags for other modern font formats
 that support font features