[selectors-4] Make empty language strings match untagged elements. #6915

Author: fantasai

selectors-4/Overview.bs

@@ -1894,7 +1894,7 @@ The Language Pseudo-class: '':lang()''</h3>
 	it is possible to write selectors that
 	represent an element based on its <a>content language</a>.
 	The <dfn id='lang-pseudo'>:lang()</dfn> pseudo-class,
-	which accepts a comma-separated list of one or more <a>language ranges</a>,
+	which accepts a comma-separated list of <a>language ranges</a>,
 	represents an element whose [=content language=]
 	is one of the languages listed in its argument.
 	Each <dfn>language range</dfn> in '':lang()''
@@ -1915,10 +1915,17 @@ The Language Pseudo-class: '':lang()''</h3>
 	when represented in BCP 47 syntax [[BCP47]],
 	it matches that <a>language range</a> in an <var>extended filtering</var>
 	operation per [[RFC4647]] <cite>Matching of Language Tags</cite> (section 3.3.2).
+	For this purpose, a wildcard [=language range=] (<code>"*"</code>) does not match
+	elements whose language is not tagged (e.g. <code>lang=""</code>),
+	but does match elements whose language is tagged as undetermined (<code>lang=und</code>).
 	The matching is performed [=ASCII case-insensitively=].
 	The <a>language range</a> does not need to be a valid language code to
 	perform this comparison.
 
+	A [=language range=] consisting of an empty string
+	(as in '':lang()'' or '':lang("")'')
+	matches (only) elements whose language is not tagged.
+
 	Note: It is recommended that documents and protocols
 	indicate language using codes from [[BCP47]] or its successor,
 	and in the case of XML-based formats, by means of <code>xml:lang</code> attributes. [[XML10]]