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.cpp

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2026, Callum Law <callumlaw1709@outlook.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CounterStyleSystemStyleValue.h"
+#include <LibWeb/CSS/Serialize.h>
+
+namespace Web::CSS {
+
+void CounterStyleSystemStyleValue::serialize(StringBuilder& builder, SerializationMode mode) const
+{
+    m_value.visit(
+        [&](CounterStyleSystem const& system) {
+            builder.append(CSS::to_string(system));
+        },
+        [&](Fixed const& fixed) {
+            builder.append("fixed"sv);
+            if (fixed.first_symbol) {
+                builder.append(' ');
+                fixed.first_symbol->serialize(builder, mode);
+            }
+        },
+        [&](Extends const& extends) {
+            builder.append("extends "sv);
+            serialize_an_identifier(builder, extends.name);
+        });
+}
+
+ValueComparingNonnullRefPtr<StyleValue const> CounterStyleSystemStyleValue::absolutized(ComputationContext const& context) const
+{
+    return m_value.visit(
+        [&](CounterStyleSystem const&) -> ValueComparingNonnullRefPtr<StyleValue const> {
+            return *this;
+        },
+        [&](Fixed const& fixed) -> ValueComparingNonnullRefPtr<StyleValue const> {
+            if (!fixed.first_symbol)
+                return *this;
+
+            auto const& absolutized_value = fixed.first_symbol->absolutized(context);
+
+            if (absolutized_value == fixed.first_symbol)
+                return *this;
+
+            return CounterStyleSystemStyleValue::create_fixed(absolutized_value);
+        },
+        [&](Extends const&) -> ValueComparingNonnullRefPtr<StyleValue const> {
+            return *this;
+        });
+}
+
+bool CounterStyleSystemStyleValue::algorithm_differs_from(CounterStyleSystemStyleValue const& other) const
+{
+    if (m_value.index() != other.m_value.index())
+        return true;
+
+    return m_value.visit(
+        [&](CounterStyleSystem const& system) -> bool {
+            return system != other.m_value.get<CounterStyleSystem>();
+        },
+        [&](Fixed const&) -> bool {
+            return false;
+        },
+        [&](Extends const& extends) -> bool {
+            // FIXME: We don't know which counter style the 'extends' refers to here, so we have to assume it might
+            //        differ if the names differ. Is this correct?
+            return extends.name != other.m_value.get<Extends>().name;
+        });
+}
+
+}