LibWeb/HTML: Update radio group when a checked radio button is inserted

Corresponds to whatwg/html#10917

Author: AtkinsSJ

Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -1649,6 +1649,19 @@ void HTMLInputElement::clear_algorithm()
 void HTMLInputElement::form_associated_element_was_inserted()
 {
     create_shadow_tree_if_needed();
+
+    // https://html.spec.whatwg.org/multipage/input.html#radio-button-state-(type=radio)
+    // When any of the following phenomena occur, if the element's checkedness state is true after the occurrence,
+    // the checkedness state of all the other elements in the same radio button group must be set to false:
+    // ...
+    // - The element becomes connected.
+    if (type_state() == TypeAttributeState::RadioButton && checked()) {
+        root().for_each_in_inclusive_subtree_of_type<HTMLInputElement>([&](auto& element) {
+            if (element.checked() && &element != this && is_in_same_radio_button_group(*this, element))
+                element.set_checked(false);
+            return TraversalDecision::Continue;
+        });
+    }
 }
 
 void HTMLInputElement::form_associated_element_was_removed(DOM::Node*)