The reference implementation for the alert widget.
This demonstrates the (contrived) use of the alert widget through a simple game: guess the number. Left clicking or typing keyboard enter/return when the button labeled "guess" is focused will cause an alert giving feedback on the textfield contents to be displayed. If you are using Firefox 1.5, the alert will be focused.
The suggested coding practice is to indicate the alert metaphor by focusing an element with an "alert" role and an "important" state set. When an element with the "alert" role attribute is focused without an "important" state set, it is should be considered an old alert and no longer worth special action/interruption/notification.