LibWeb: Parse @counter-style system descriptor

Author: https://github.com/Calme1709
Commit: https://github.com/LadybirdBrowser/ladybird/commit/f60bfd9e9e2
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/7708
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/StyleValues/CounterStyleSystemStyleValue.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2026, Callum Law <callumlaw1709@outlook.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/Enums.h>
+#include <LibWeb/CSS/StyleValues/StyleValue.h>
+
+namespace Web::CSS {
+
+class CounterStyleSystemStyleValue : public StyleValueWithDefaultOperators<CounterStyleSystemStyleValue> {
+public:
+    NonnullRefPtr<StyleValue const> static create(CounterStyleSystem system)
+    {
+        return adopt_ref(*new CounterStyleSystemStyleValue(system));
+    }
+
+    NonnullRefPtr<StyleValue const> static create_fixed(RefPtr<StyleValue const> first_symbol)
+    {
+        return adopt_ref(*new CounterStyleSystemStyleValue(Fixed { move(first_symbol) }));
+    }
+
+    NonnullRefPtr<StyleValue const> static create_extends(FlyString name)
+    {
+        return adopt_ref(*new CounterStyleSystemStyleValue(Extends { move(name) }));
+    }
+
+    virtual ~CounterStyleSystemStyleValue() override = default;
+
+    virtual void serialize(StringBuilder& builder, SerializationMode mode) const override;
+    virtual ValueComparingNonnullRefPtr<StyleValue const> absolutized(ComputationContext const& context) const override;
+    bool algorithm_differs_from(CounterStyleSystemStyleValue const& other) const;
+
+    bool properties_equal(CounterStyleSystemStyleValue const& other) const { return m_value == other.m_value; }
+
+private:
+    struct Fixed {
+        ValueComparingRefPtr<StyleValue const> first_symbol;
+        bool operator==(Fixed const&) const = default;
+    };
+
+    struct Extends {
+        FlyString name;
+        bool operator==(Extends const&) const = default;
+    };
+
+    explicit CounterStyleSystemStyleValue(Variant<CounterStyleSystem, Fixed, Extends> value)
+        : StyleValueWithDefaultOperators(Type::CounterStyleSystem)
+        , m_value(move(value))
+    {
+    }
+
+    Variant<CounterStyleSystem, Fixed, Extends> m_value;
+};
+
+}