LibWeb/CSS: Implement pending-substitution value

This is a special value temporarily assigned to longhands when their
shorthand is unresolved, and then later replaced once that is resolved.

Libraries/LibWeb/CSS/StyleValues/PendingSubstitutionStyleValue.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSStyleValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.csswg.org/css-values-5/#pending-substitution-value
+class PendingSubstitutionStyleValue final : public StyleValueWithDefaultOperators<PendingSubstitutionStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<PendingSubstitutionStyleValue> create()
+    {
+        static ValueComparingNonnullRefPtr<PendingSubstitutionStyleValue> instance = adopt_ref(*new (nothrow) PendingSubstitutionStyleValue());
+        return instance;
+    }
+    virtual ~PendingSubstitutionStyleValue() override = default;
+    virtual String to_string(SerializationMode) const override { return {}; }
+
+    // We shouldn't need to compare these, but in case we do: The nature of them is that their value is unknown, so
+    // consider them all to be unique.
+    bool properties_equal(PendingSubstitutionStyleValue const&) const { return false; }
+
+private:
+    PendingSubstitutionStyleValue()
+        : StyleValueWithDefaultOperators(Type::PendingSubstitution)
+    {
+    }
+};
+
+}