LibWeb/DOM: Introduce an ElementReference type

We have the "Element, but also maybe a pseudo-element of it" concept in
a lot of places, so let's wrap it up in a single type to make it easier
to deal with.

Libraries/LibWeb/DOM/ElementReference.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGC/Cell.h>
+#include <LibWeb/CSS/Selector.h>
+#include <LibWeb/DOM/Element.h>
+
+namespace Web::DOM {
+
+class ElementReference {
+public:
+    ElementReference(GC::Ref<Element> element, Optional<CSS::Selector::PseudoElement::Type> pseudo_element = {})
+        : m_element(element)
+        , m_pseudo_element(move(pseudo_element))
+    {
+    }
+
+    Element& element() { return m_element; }
+    Element const& element() const { return m_element; }
+    Optional<CSS::Selector::PseudoElement::Type> pseudo_element() const { return m_pseudo_element; }
+
+    void visit(GC::Cell::Visitor& visitor) const
+    {
+        visitor.visit(m_element);
+    }
+
+private:
+    GC::Ref<Element> m_element;
+    Optional<CSS::Selector::PseudoElement::Type> m_pseudo_element;
+};
+
+}