LibWeb/CSS: Implement CSSMathValue

This is a base class for the various math functions, so it's not used
directly anywhere.

Libraries/LibWeb/CSS/CSSMathValue.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/CSSMathValuePrototype.h>
+#include <LibWeb/CSS/CSSNumericValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathvalue
+class CSSMathValue : public CSSNumericValue {
+    WEB_PLATFORM_OBJECT(CSSMathValue, CSSNumericValue);
+    GC_DECLARE_ALLOCATOR(CSSMathValue);
+
+public:
+    virtual ~CSSMathValue() override = default;
+
+    Bindings::CSSMathOperator operator_() const { return m_operator; }
+
+    enum class Nested : u8 {
+        No,
+        Yes,
+    };
+    enum class Parens : u8 {
+        With,
+        Without,
+    };
+    virtual String serialize_math_value(Nested, Parens) const = 0;
+
+protected:
+    explicit CSSMathValue(JS::Realm&, Bindings::CSSMathOperator, NumericType);
+
+    virtual void initialize(JS::Realm&) override;
+
+    Bindings::CSSMathOperator m_operator;
+};
+
+}