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.
Obsolete since Gecko 51 (Firefox 51 / Thunderbird 51 / SeaMonkey 2.48)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
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.