diff --git a/Libraries/LibWeb/DOM/Element.cpp b/Libraries/LibWeb/DOM/Element.cpp
index e8fc97f6ac3..1b023d32555 100644
--- a/Libraries/LibWeb/DOM/Element.cpp
+++ b/Libraries/LibWeb/DOM/Element.cpp
@@ -46,6 +46,7 @@
 #include <LibWeb/HTML/CustomElements/CustomElementName.h>
 #include <LibWeb/HTML/CustomElements/CustomElementReactionNames.h>
 #include <LibWeb/HTML/CustomElements/CustomElementRegistry.h>
+#include <LibWeb/HTML/CustomElements/CustomStateSet.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
 #include <LibWeb/HTML/HTMLAreaElement.h>
@@ -118,6 +119,7 @@ void Element::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_class_list);
     visitor.visit(m_shadow_root);
     visitor.visit(m_custom_element_definition);
+    visitor.visit(m_custom_state_set);
     visitor.visit(m_cascaded_properties);
     visitor.visit(m_computed_properties);
     if (m_pseudo_element_data) {
@@ -3824,6 +3826,13 @@ auto Element::ensure_custom_element_reaction_queue() -> CustomElementReactionQue
     return *m_custom_element_reaction_queue;
 }
 
+HTML::CustomStateSet& Element::ensure_custom_state_set()
+{
+    if (!m_custom_state_set)
+        m_custom_state_set = HTML::CustomStateSet::create(realm(), *this);
+    return *m_custom_state_set;
+}
+
 CSS::StyleSheetList& Element::document_or_shadow_root_style_sheets()
 {
     auto& root_node = root();
diff --git a/Libraries/LibWeb/DOM/Element.h b/Libraries/LibWeb/DOM/Element.h
index ebe0e4e61b8..6941e7c9283 100644
--- a/Libraries/LibWeb/DOM/Element.h
+++ b/Libraries/LibWeb/DOM/Element.h
@@ -351,6 +351,9 @@ public:
     CustomElementReactionQueue const* custom_element_reaction_queue() const { return m_custom_element_reaction_queue; }
     CustomElementReactionQueue& ensure_custom_element_reaction_queue();
 
+    HTML::CustomStateSet const* custom_state_set() const { return m_custom_state_set; }
+    HTML::CustomStateSet& ensure_custom_state_set();
+
     JS::ThrowCompletionOr<void> upgrade_element(GC::Ref<HTML::CustomElementDefinition> custom_element_definition);
     void try_to_upgrade();
 
@@ -587,6 +590,9 @@ private:
     // https://dom.spec.whatwg.org/#concept-element-is-value
     Optional<String> m_is_value;
 
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#states-set
+    GC::Ptr<HTML::CustomStateSet> m_custom_state_set;
+
     // https://www.w3.org/TR/intersection-observer/#dom-element-registeredintersectionobservers-slot
     // Element objects have an internal [[RegisteredIntersectionObservers]] slot, which is initialized to an empty list.
     OwnPtr<Vector<IntersectionObserver::IntersectionObserverRegistration>> m_registered_intersection_observers;
diff --git a/Libraries/LibWeb/HTML/ElementInternals.cpp b/Libraries/LibWeb/HTML/ElementInternals.cpp
index 988e7bfe01d..53479232e50 100644
--- a/Libraries/LibWeb/HTML/ElementInternals.cpp
+++ b/Libraries/LibWeb/HTML/ElementInternals.cpp
@@ -208,6 +208,13 @@ WebIDL::ExceptionOr<GC::Ptr<DOM::NodeList>> ElementInternals::labels()
     return WebIDL::NotSupportedError::create(realm(), "FIXME: ElementInternals::labels()"_string);
 }
 
+// https://html.spec.whatwg.org/multipage/custom-elements.html#dom-elementinternals-states
+GC::Ptr<CustomStateSet> ElementInternals::states()
+{
+    // The states getter steps are to return this's target element's states set.
+    return m_target_element->ensure_custom_state_set();
+}
+
 void ElementInternals::initialize(JS::Realm& realm)
 {
     WEB_SET_PROTOTYPE_FOR_INTERFACE(ElementInternals);
diff --git a/Libraries/LibWeb/HTML/ElementInternals.h b/Libraries/LibWeb/HTML/ElementInternals.h
index e57e086462c..150a423fed9 100644
--- a/Libraries/LibWeb/HTML/ElementInternals.h
+++ b/Libraries/LibWeb/HTML/ElementInternals.h
@@ -57,6 +57,7 @@ public:
     WebIDL::ExceptionOr<bool> report_validity() const;
 
     WebIDL::ExceptionOr<GC::Ptr<DOM::NodeList>> labels();
+    GC::Ptr<CustomStateSet> states();
 
 private:
     explicit ElementInternals(JS::Realm&, HTMLElement& target_element);
diff --git a/Libraries/LibWeb/HTML/ElementInternals.idl b/Libraries/LibWeb/HTML/ElementInternals.idl
index 34912a2e2f0..be95b5ebd80 100644
--- a/Libraries/LibWeb/HTML/ElementInternals.idl
+++ b/Libraries/LibWeb/HTML/ElementInternals.idl
@@ -1,4 +1,5 @@
 #import <DOM/ShadowRoot.idl>
+#import <HTML/CustomElements/CustomStateSet.idl>
 
 // https://html.spec.whatwg.org/multipage/custom-elements.html#elementinternals
 [Exposed=Window]
@@ -24,7 +25,7 @@ interface ElementInternals {
     readonly attribute NodeList labels;
 
     // Custom state pseudo-class
-    [FIXME, SameObject] readonly attribute CustomStateSet states;
+    [SameObject] readonly attribute CustomStateSet states;
 };
 
 // Accessibility semantics
diff --git a/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/ElementInternals-states.txt b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/ElementInternals-states.txt
new file mode 100644
index 00000000000..bb8a4e0ba5d
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/ElementInternals-states.txt
@@ -0,0 +1,9 @@
+Harness status: OK
+
+Found 4 tests
+
+4 Pass
+Pass	CustomStateSet behavior of ElementInternals.states: Initial state
+Pass	CustomStateSet behavior of ElementInternals.states: Exceptions
+Pass	CustomStateSet behavior of ElementInternals.states: Modifications
+Pass	Updating a CustomStateSet while iterating it should work
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/custom-state-set-strong-ref.txt b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/custom-state-set-strong-ref.txt
new file mode 100644
index 00000000000..c6818539218
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/state/custom-state-set-strong-ref.txt
@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	customstateset doesn't crash after GC on detached node
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/common/gc.js b/Tests/LibWeb/Text/input/wpt-import/common/gc.js
new file mode 100644
index 00000000000..ac43a4cfaf7
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/common/gc.js
@@ -0,0 +1,52 @@
+/**
+ * Does a best-effort attempt at invoking garbage collection. Attempts to use
+ * the standardized `TestUtils.gc()` function, but falls back to other
+ * environment-specific nonstandard functions, with a final result of just
+ * creating a lot of garbage (in which case you will get a console warning).
+ *
+ * This should generally only be used to attempt to trigger bugs and crashes
+ * inside tests, i.e. cases where if garbage collection happened, then this
+ * should not trigger some misbehavior. You cannot rely on garbage collection
+ * successfully trigger, or that any particular unreachable object will be
+ * collected.
+ *
+ * @returns {Promise<undefined>} A promise you should await to ensure garbage
+ * collection has had a chance to complete.
+ */
+self.garbageCollect = async () => {
+  // https://testutils.spec.whatwg.org/#the-testutils-namespace
+  if (self.TestUtils?.gc) {
+    return TestUtils.gc();
+  }
+
+  // Use --expose_gc for V8 (and Node.js)
+  // to pass this flag at chrome launch use: --js-flags="--expose-gc"
+  // Exposed in SpiderMonkey shell as well
+  if (self.gc) {
+    return self.gc();
+  }
+
+  // Present in some WebKit development environments
+  if (self.GCController) {
+    return GCController.collect();
+  }
+
+  console.warn(
+    'Tests are running without the ability to do manual garbage collection. ' +
+    'They will still work, but coverage will be suboptimal.');
+
+  for (var i = 0; i < 1000; i++) {
+    gcRec(10);
+  }
+
+  function gcRec(n) {
+    if (n < 1) {
+      return {};
+    }
+
+    let temp = { i: "ab" + i + i / 100000 };
+    temp += "foo";
+
+    gcRec(n - 1);
+  }
+};
diff --git a/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/ElementInternals-states.html b/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/ElementInternals-states.html
new file mode 100644
index 00000000000..f73260cd546
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/ElementInternals-states.html
@@ -0,0 +1,83 @@
+<!DOCTYPE html>
+<link rel=help href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script>
+class TestElement extends HTMLElement {
+  constructor() {
+    super();
+    this._internals = this.attachInternals();
+  }
+
+  get internals() {
+    return this._internals;
+  }
+}
+customElements.define("test-element", TestElement);
+
+test(() => {
+  let i = (new TestElement()).internals;
+
+  assert_true(i.states instanceof CustomStateSet);
+  assert_equals(i.states.size, 0);
+  assert_false(i.states.has('foo'));
+  assert_false(i.states.has('--foo'));
+  assert_equals(i.states.toString(), '[object CustomStateSet]');
+}, 'CustomStateSet behavior of ElementInternals.states: Initial state');
+
+test(() => {
+  let i = (new TestElement()).internals;
+  assert_throws_js(TypeError, () => { i.states.supports('foo'); });
+  i.states.add(''); // should not throw.
+  i.states.add('--a\tb'); // should not throw.
+}, 'CustomStateSet behavior of ElementInternals.states: Exceptions');
+
+test(() => {
+  let i = (new TestElement()).internals;
+  i.states.add('--foo');
+  i.states.add('--bar');
+  i.states.add('--foo');
+  assert_equals(i.states.size, 2);
+  assert_true(i.states.has('--foo'));
+  assert_true(i.states.has('--bar'));
+  assert_array_equals([...i.states], ['--foo', '--bar']);
+  i.states.delete('--foo');
+  assert_array_equals([...i.states], ['--bar']);
+  i.states.add('--foo');
+  assert_array_equals([...i.states], ['--bar', '--foo']);
+  i.states.delete('--bar');
+  i.states.add('--baz');
+  assert_array_equals([...i.states], ['--foo', '--baz']);
+}, 'CustomStateSet behavior of ElementInternals.states: Modifications');
+
+test(() => {
+  let i = (new TestElement()).internals;
+  i.states.add('--one');
+  i.states.add('--two');
+  i.states.add('--three');
+  let iter = i.states.values();
+
+  // Delete the next item.
+  i.states.delete('--one');
+  let item = iter.next();
+  assert_false(item.done);
+  assert_equals(item.value, '--two');
+
+  // Clear the set.
+  i.states.clear();
+  item = iter.next();
+  assert_true(item.done);
+
+  // Delete the previous item.
+  i.states.add('--one');
+  i.states.add('--two');
+  i.states.add('--three');
+  iter = i.states.values();
+  item = iter.next();
+  assert_equals(item.value, '--one');
+  i.states.delete('--one');
+  item = iter.next();
+  assert_equals(item.value, '--two');
+}, 'Updating a CustomStateSet while iterating it should work');
+</script>
+
diff --git a/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/custom-state-set-strong-ref.html b/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/custom-state-set-strong-ref.html
new file mode 100644
index 00000000000..d555da62e60
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/custom-elements/state/custom-state-set-strong-ref.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <meta name="timeout" content="long">
+    <meta name="author" title="Keith Cirkel" href="mailto:wpt@keithcirkel.co.uk" />
+    <link rel="help" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class" />
+    <title>CustomStateSet doesn't crash after GC on detached node</title>
+
+    <script src="../../resources/testharness.js"></script>
+    <script src="../../resources/testharnessreport.js"></script>
+    <script src="../../common/gc.js"></script>
+  </head>
+  <body>
+    <custom-state id="myCE"></custom-state>
+    <script>
+    customElements.define('custom-state', class extends HTMLElement {
+      connectedCallback() {
+        this.elementInternals = this.attachInternals();
+      }
+    });
+
+    promise_test(async function() {
+      const states = myCE.elementInternals.states;
+      myCE.remove();
+      await garbageCollect();
+      states.add('still-works');
+      assert_equals(states.size, 1);
+      assert_true(states.delete('still-works'));
+      assert_equals(states.size, 0);
+    }, "customstateset doesn't crash after GC on detached node");
+    </script>
+  </body>
+</html>