A few tools that I’m working on listen the click event and then show a border or outline around an element. This lets you see the size easily and make changes. But the click event is not stopped and things like checkboxes are selected.
Web Developer tools in most browsers have a similar “inspect” tool that helps lets you click on a item on the page and not trigger any event.
Ferdinand is writing a HTML inspector tool and he wants to prevent the default event and event propagation. He uses the event.stopPropagation methods.