This event is emitted before an existing element is kept during
a page update.
Event listeners can call event.preventDefault() on an up:fragment:keep event
to prevent the element from being persisted. If the event is prevented, the element
will be replaced by a fragment from the response.
Event listeners may call this method to prevent the element from being preserved.
The fragment that will be kept.
The discarded element.
The value of the up-data attribute of the discarded element,
parsed as a JSON object.