LibWeb: Use existing StyleValues for math-depth

Previously we implemented an all encompassing `MathDepthStyleValue`
specifically for the `math-depth` property, this was unnecessary since
we can represent `auto-add` and `<integer>` using existing `StyleValue`
classes.

This brings the values created from parsing in line with those set via
`StylePropertyMap` which allows us to simplify computation

Libraries/LibWeb/CSS/StyleValues/AddFunctionStyleValue.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2026, Callum Law <callumlaw1709@outlook.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/StyleValues/StyleValue.h>
+
+namespace Web::CSS {
+
+class AddFunctionStyleValue : public StyleValueWithDefaultOperators<AddFunctionStyleValue> {
+public:
+    static NonnullRefPtr<AddFunctionStyleValue> create(NonnullRefPtr<StyleValue const> value)
+    {
+        return adopt_ref(*new AddFunctionStyleValue(move(value)));
+    }
+
+    NonnullRefPtr<StyleValue const> value() const { return m_value; }
+    virtual void serialize(StringBuilder&, SerializationMode) const override;
+
+    bool properties_equal(AddFunctionStyleValue const& other) const { return m_value == other.m_value; }
+
+private:
+    AddFunctionStyleValue(NonnullRefPtr<StyleValue const> value)
+        : StyleValueWithDefaultOperators(Type::AddFunction)
+        , m_value(move(value))
+    {
+    }
+
+    virtual ~AddFunctionStyleValue() override = default;
+
+    ValueComparingNonnullRefPtr<StyleValue const> m_value;
+};
+
+}