[selectors-4] Make empty language strings match untagged elements. w3c#6915

Author: fantasai

selectors-4/Overview.bs

@@ -1915,10 +1915,17 @@ The Language Pseudo-class: '':lang()''
 	when represented in BCP 47 syntax [[BCP47]],
 	it matches that language range in an extended filtering
 	operation per [[RFC4647]] Matching of Language Tags (section 3.3.2).
+	For this purpose, a wildcard [=language range=] ("*") does not match
+	elements whose language is not tagged (e.g. lang=""),
+	but does match elements whose language is tagged as undetermined (lang=und).
 	The matching is performed [=ASCII case-insensitively=].
 	The language range does not need to be a valid language code to
 	perform this comparison.
 
+	A [=language range=] consisting of an empty string
+	('':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 xml:lang attributes. [[XML10]]