LibWeb: Add CustomStateSet IDL type

Author: https://github.com/AtkinsSJ
Commit: e63d81b36e
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5302
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/HTML/CustomElements/CustomStateSet.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Set.h>
+#include <LibJS/Runtime/SetIterator.h>
+#include <LibWeb/Bindings/CustomStateSetPrototype.h>
+#include <LibWeb/Bindings/PlatformObject.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/custom-elements.html#customstateset
+class CustomStateSet final : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(CustomStateSet, Bindings::PlatformObject);
+    GC_DECLARE_ALLOCATOR(CustomStateSet);
+
+public:
+    [[nodiscard]] static GC::Ref<CustomStateSet> create(JS::Realm&, GC::Ref<DOM::Element>);
+    virtual ~CustomStateSet() override = default;
+
+    GC::Ref<JS::Set> set_entries() const { return m_set_entries; }
+    bool has_state(FlyString const&) const;
+
+    void on_set_modified_from_js(Badge<Bindings::CustomStateSetPrototype>);
+
+private:
+    CustomStateSet(JS::Realm&, GC::Ref<DOM::Element>);
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    GC::Ref<JS::Set> m_set_entries;
+    GC::Ref<DOM::Element> m_element;
+};
+
+}