LibWeb/CSS: Implement "Create a sum value"

Author: https://github.com/AtkinsSJ
Commit: 80abffd2e8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6162
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/CSS/CSSMathMax.cpp

@@ -129,4 +129,32 @@ bool CSSMathMax::is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const
     return m_values->is_equal_numeric_values(other_max->m_values);
 }
 
+// https://drafts.css-houdini.org/css-typed-om-1/#create-a-sum-value
+Optional<SumValue> CSSMathMax::create_a_sum_value() const
+{
+    // 1. Let args be the result of creating a sum value for each item in this’s values internal slot.
+    Vector<Optional<SumValue>> args;
+    args.ensure_capacity(m_values->length());
+    for (auto const& value : m_values->values()) {
+        args.unchecked_append(value->create_a_sum_value());
+    }
+
+    Optional<SumValue> item_with_largest_value = args.first();
+    for (auto const& item : args) {
+        // 2. If any item of args is failure, or has a length greater than one, return failure.
+        if (!item.has_value() || item->size() > 1)
+            return {};
+
+        // 3. If not all of the unit maps among the items of args are identical, return failure.
+        if (item->first().unit_map != item_with_largest_value->first().unit_map)
+            return {};
+
+        if (item->first().value > item_with_largest_value->first().value)
+            item_with_largest_value = item;
+    }
+
+    // 4. Return the item of args whose sole item has the largest value.
+    return item_with_largest_value;
+}
+
 }