LibWeb: Implement the math-depth CSS property

This one is a bit fun because it can be `add(<integer>)` or `auto-add`,
but children have to inherit the computed value not the specified one.
We also have to compute it before computing the font-size, because of
`font-size: math` which will be implemented later.

Userland/Libraries/LibWeb/CSS/StyleValues/MathDepthStyleValue.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+class MathDepthStyleValue : public StyleValueWithDefaultOperators<MathDepthStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<MathDepthStyleValue> create_auto_add();
+    static ValueComparingNonnullRefPtr<MathDepthStyleValue> create_add(ValueComparingNonnullRefPtr<StyleValue const> integer_value);
+    static ValueComparingNonnullRefPtr<MathDepthStyleValue> create_integer(ValueComparingNonnullRefPtr<StyleValue const> integer_value);
+    virtual ~MathDepthStyleValue() override = default;
+
+    bool is_auto_add() const { return m_type == MathDepthType::AutoAdd; }
+    bool is_add() const { return m_type == MathDepthType::Add; }
+    bool is_integer() const { return m_type == MathDepthType::Integer; }
+    auto integer_value() const
+    {
+        VERIFY(!m_integer_value.is_null());
+        return m_integer_value;
+    }
+    virtual String to_string() const override;
+
+    bool properties_equal(MathDepthStyleValue const& other) const;
+
+private:
+    enum class MathDepthType {
+        AutoAdd,
+        Add,
+        Integer,
+    };
+
+    MathDepthStyleValue(MathDepthType type, ValueComparingRefPtr<StyleValue const> integer_value = nullptr);
+
+    MathDepthType m_type;
+    ValueComparingRefPtr<StyleValue const> m_integer_value;
+};
+
+}