diff --git a/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Libraries/LibWeb/CSS/SelectorEngine.cpp
index 80ae1ae204a..66fa7649b7b 100644
--- a/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -53,6 +53,67 @@ static inline GC::Ptr<DOM::Node const> traverse_up(GC::Ptr<DOM::Node const> node
     return node->parent();
 }
 
+// https://www.rfc-editor.org/rfc/rfc4647.html#section-3.3.2
+// NB: Language tags only use ASCII characters, so we can get away with using StringView.
+static bool language_range_matches_tag(StringView language_range, StringView language_tag)
+{
+    // 1. Split both the extended language range and the language tag being compared into a list of subtags by
+    //    dividing on the hyphen (%x2D) character.
+    auto range_subtags = language_range.split_view('-', SplitBehavior::KeepEmpty);
+    auto tag_subtags = language_tag.split_view('-', SplitBehavior::KeepEmpty);
+
+    //    Two subtags match if either they are the same when compared case-insensitively or the language range's subtag
+    //    is the wildcard '*'.
+    auto subtags_match = [](StringView language_range_subtag, StringView language_subtag) {
+        return language_range_subtag == "*"sv
+            || Infra::is_ascii_case_insensitive_match(language_range_subtag, language_subtag);
+    };
+
+    // 2. Begin with the first subtag in each list. If the first subtag in the range does not match the first
+    //    subtag in the tag, the overall match fails. Otherwise, move to the next subtag in both the range and the
+    //    tag.
+    auto tag_subtag = tag_subtags.begin();
+    auto range_subtag = range_subtags.begin();
+    if (!subtags_match(*range_subtag, *tag_subtag))
+        return false;
+    ++tag_subtag;
+    ++range_subtag;
+
+    // 3. While there are more subtags left in the language range's list:
+    while (!range_subtag.is_end()) {
+        // A. If the subtag currently being examined in the range is the wildcard ('*'), move to the next subtag in
+        //    the range and continue with the loop.
+        if (*range_subtag == "*"sv) {
+            ++range_subtag;
+            continue;
+        }
+
+        // B. Else, if there are no more subtags in the language tag's list, the match fails.
+        if (tag_subtag.is_end())
+            return false;
+
+        // C. Else, if the current subtag in the range's list matches the current subtag in the language tag's
+        //    list, move to the next subtag in both lists and continue with the loop.
+        if (subtags_match(*range_subtag, *tag_subtag)) {
+            ++range_subtag;
+            ++tag_subtag;
+            continue;
+        }
+
+        // D. Else, if the language tag's subtag is a "singleton" (a single letter or digit, which includes the
+        //    private-use subtag 'x') the match fails.
+        if (tag_subtag->length() == 1 && is_ascii_alphanumeric((*tag_subtag)[0])) {
+            return false;
+        }
+
+        // E. Else, move to the next subtag in the language tag's list and continue with the loop.
+        ++tag_subtag;
+    }
+
+    // 4. When the language range's list has no more subtags, the match succeeds.
+    return true;
+}
+
 // https://drafts.csswg.org/selectors-4/#the-lang-pseudo
 static inline bool matches_lang_pseudo_class(DOM::Element const& element, Vector<FlyString> const& languages)
 {
@@ -62,21 +123,17 @@ static inline bool matches_lang_pseudo_class(DOM::Element const& element, Vector
 
     auto element_language = maybe_element_language.release_value();
 
-    // FIXME: This is ad-hoc. Implement a proper language range matching algorithm as recommended by BCP47.
-    for (auto const& language : languages) {
-        if (language.is_empty())
-            continue;
-        if (language == "*"sv)
+    // The element’s content language matches a language range if its content language, as represented in BCP 47
+    // syntax, matches the given language range in an extended filtering operation per [RFC4647] Matching of Language
+    // Tags (section 3.3.2). Both the content language and the language range must be canonicalized and converted to
+    // extlang form as per section 4.5 of [RFC5646] prior to the extended filtering operation. The matching is
+    // performed case-insensitively within the ASCII range.
+
+    // FIXME: Canonicalize both as extlang.
+
+    for (auto const& language_range : languages) {
+        if (language_range_matches_tag(language_range, element_language))
             return true;
-        auto element_language_length = element_language.bytes_as_string_view().length();
-        auto language_length = language.bytes_as_string_view().length();
-        if (element_language_length == language_length) {
-            if (Infra::is_ascii_case_insensitive_match(element_language, language))
-                return true;
-        } else if (element_language_length > language_length) {
-            if (element_language.starts_with_bytes(language, CaseSensitivity::CaseInsensitive) && element_language.bytes_as_string_view()[language_length] == '-')
-                return true;
-        }
     }
     return false;
 }
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html
new file mode 100644
index 00000000000..466e9f464b9
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching reference</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+
+<style>
+div.test { color: green; }
+</style>
+
+<div class="test">This should be green</div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-000.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-000.html
new file mode 100644
index 00000000000..994ef4ae23e
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-000.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+:lang(en-US) { color: green; }
+</style>
+
+<div class="test">This should be green</div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-001.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-001.html
new file mode 100644
index 00000000000..3f8686e5907
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-001.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang(fr) { color: green; }
+</style>
+
+<div class="test"><span lang="fr">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-002.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-002.html
new file mode 100644
index 00000000000..769f924ce61
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-002.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("FR") { color: green; }
+</style>
+
+<div class="test"><span lang="fr">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-003.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-003.html
new file mode 100644
index 00000000000..55fc15fa2bc
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-003.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("Fr") { color: green; }
+</style>
+
+<div class="test"><span lang="fR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-004.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-004.html
new file mode 100644
index 00000000000..e0e916e799b
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-004.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("fr") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-CH">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-005.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-005.html
new file mode 100644
index 00000000000..a1d0b037292
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-005.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: green; }
+:lang("fr-CH") { color: red; }
+</style>
+
+<div class="test"><span lang="fr">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-006.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-006.html
new file mode 100644
index 00000000000..92d0199a505
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-006.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: green; }
+:lang("fr-CH") { color: red; }
+</style>
+
+<div class="test"><span lang="fr-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-007.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-007.html
new file mode 100644
index 00000000000..9e4e1b8dfd1
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-007.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("*-CH") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-CH">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-008.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-008.html
new file mode 100644
index 00000000000..01e1379dfda
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-008.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("*-Latn") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-009.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-009.html
new file mode 100644
index 00000000000..53d1d5d4588
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-009.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("fr-FR") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-010.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-010.html
new file mode 100644
index 00000000000..c0775a2204f
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-010.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("*-FR") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-011.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-011.html
new file mode 100644
index 00000000000..7288043f39b
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-011.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("fr", "nl", "de") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-012.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-012.html
new file mode 100644
index 00000000000..3c2d08c9021
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-012.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang(de, nl, fr) { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-013.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-013.html
new file mode 100644
index 00000000000..72174ff3cdb
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-013.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: green; }
+:lang(de, nl, 0, fr) { color: red; }
+</style>
+
+<div class="test"><span lang="fr">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-014.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-014.html
new file mode 100644
index 00000000000..490785fbf65
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-014.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: green; }
+:lang(0) { color: red; }
+</style>
+
+<div class="test"><span lang="0">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-015.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-015.html
new file mode 100644
index 00000000000..01bc0b6df2a
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-015.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang(\*-FR) { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-016.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-016.html
new file mode 100644
index 00000000000..ec90c73126a
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-016.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang(fr) { color: green; }
+</style>
+
+<div class="test"><span lang="fr-FR-x-foobar">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-017.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-017.html
new file mode 100644
index 00000000000..f0c19666ef6
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-017.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("fr-x-foobar") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR-x-foobar">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-018.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-018.html
new file mode 100644
index 00000000000..d050461b3ab
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-018.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("*-x-foobar") { color: green; }
+</style>
+
+<div class="test"><span lang="fr-Latn-FR-x-foobar">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-019.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-019.html
new file mode 100644
index 00000000000..b0d03aaf188
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-019.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: green; }
+:lang("fr-x-foobar") { color: red; }
+</style>
+
+<div class="test"><span lang="fr">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-020.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-020.html
new file mode 100644
index 00000000000..dd90ab1fc55
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-020.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("iw") { color: green; }
+</style>
+
+<div class="test"><span lang="iw-ase-jpan-basiceng">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-021.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-021.html
new file mode 100644
index 00000000000..bde7152233a
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-021.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+span:lang("en-gb-oed") { color: magenta; }
+span span:lang("*-gb") { color: green; }
+</style>
+
+<div class="test" lang="en-GB-oed"><span><span>This should be green</span></span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-022.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-022.html
new file mode 100644
index 00000000000..e0490a827f6
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-022.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("i-navajo") { color: green; }
+</style>
+
+<div class="test"><span lang="i-navajo">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-023.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-023.html
new file mode 100644
index 00000000000..f2b9e1ecab6
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-023.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("x") { color: green; } /* not a well-formed lang tag, but matches per
+                                the Extended Filtering algorithm */
+</style>
+
+<div class="test"><span lang="x-lojban">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-024.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-024.html
new file mode 100644
index 00000000000..48ed98c8948
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-024.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("art") { color: green; }
+</style>
+
+<div class="test"><span lang="art-lojban">This should be green</span></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-025.html b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-025.html
new file mode 100644
index 00000000000..8a1f6e15669
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/selectors/selectors-4/lang-025.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html lang="en-US">
+<meta charset="utf-8">
+<title>CSS Selectors 4 - :lang matching</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#the-lang-pseudo">
+<link rel="match" href="../../../../../expected/wpt-import/css/selectors/selectors-4/lang-000-ref.html">
+
+<style>
+div.test { color: red; }
+:lang("art") { color: green; }
+</style>
+
+<!-- This can match :lang("art"), because "-x-lojban" is a private subtag,
+     so this is *not* the grandfathered "art-lojban" tag. -->
+<div class="test"><span lang="art-x-lojban">This should be green</span></div>