[selectors-4] Canonicalize into extlang form prior to :lang() matching (w3c#4212)

Fixes w3c#4154

Co-authored-by: Chris Lilley <chris@w3.org>

Author: frivoal

selectors-4/Overview.bs

@@ -27,7 +27,7 @@ At Risk: the column combinator
 At Risk: [=user action pseudo-classes=] applying to non-[=tree-abiding=] [=pseudo-elements=]
 At Risk: the '':blank'' pseudo-class
 Ignored Terms: function token, Document, DocumentFragment, math, h1, shadow tree, querySelector(), quirks mode, button, a, span, object, p, div, q, area, link, label, input, html, em, li, ol, pre, CSS Value Definition Syntax
-Ignored Vars: identifier, extended filtering, i
+Ignored Vars: identifier, i
 </pre>
 <pre class=link-defaults>
 spec:css-values-4; type:dfn; text:identifier
@@ -1977,21 +1977,24 @@ The Language Pseudo-class: '':lang()''</h3>
 		e.g. '':lang(\*-Latn)'' or '':lang("*-Latn")''.)
 
 	Note: The <a>content language</a> of an element is defined by the document language.
-	For example, in HTML [[HTML5]], the <a>content language</a> is determined
+
+  For example, in HTML [[HTML5]], the <a>content language</a> is determined
 	by a combination of the <code>lang</code> attribute,
 	information from <a element>meta</a> elements,
 	and possibly also the protocol (e.g. from HTTP headers).
 	XML languages can use the <code>xml:lang</code> attribute
 	to indicate language information for an element. [[XML10]]
 
-	An element's <a>content language</a> matches a <a>language range</a> if,
-	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 element's <a>content language</a> matches a <a>language range</a> if
+	its <a>content language</a>, as represented in BCP 47 syntax,
+	matches the given <a>language range</a> in an <i>extended filtering</i>
+	operation per [[!RFC4647]] <cite>Matching of Language Tags</cite> (section 3.3.2).
+	Both the [=content language=] and the [=language range=]
+	must be <i>canonicalized</i>
+	and converted to <i>extlang form</i> as per section 4.5 of [[!RFC5646]]
+	prior to the <i>extended filtering</i> operation.
+	The matching is performed case-insensitively within the ASCII range.
+
 	The <a>language range</a> does not need to be a valid language code to
 	perform this comparison.