This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Performs some, possibly platform-specific, action designed to get the user's attention. This might involve playing a sound effect, flashing the screen, or presenting some other form of effect.
Not part of any specification.