Error conditions

Most browsers, when configured to block cookies, will also block localStorage. Attempts to use it will result in an exception. Do not forget to manage these cases.

var video = document.querySelector('video')
try {
    video.volume = localStorage.getItem('volume')
} catch (error) {
    alert('If you\'d like your volume saved, turn on cookies')
}
video.play()

If error were not handled, program would stop functioning properly.