What happens to event propagation in Shadow DOM by default?
By default, only events with the composed flag set to true can cross Shadow DOM boundaries during propagation. Built-in events like 'click', 'focus', and 'blur' are composed by default, while custom events are not composed unless explicitly set. This boundary behavior is a key aspect of Shadow DOM encapsulation, ensuring that internal events stay internal unless specifically designed to propagate outside. Understanding this is crucial when working with Web Components and custom elements.