[css-fonts-4] Clarify precedence of font-presentation versus lang/xml:lang

w3c#2138

Author: Litherum

css-fonts-4/Overview.bs

@@ -3641,6 +3641,9 @@ If one of these variation selectors is present,
 but unsupported by the font previously selected,
 the variation selector is ignored.
 
+BCP47's <code>-u-</code> extension to the language tag accepted by <code>lang</code> or <code>xml:lang</code> should not be
+considered when the user-agent decides whether to use emoji presentation or text presentation for a particular character.
+
 <dl dfn-for=font-variant-emoji dfn-type=value>
 	<dt><dfn>auto</dfn>
 	<dd>