When a page displays a notification, additionally "draws attention" to the containing window.
The exact effect of this extension is platform-specific. To quote the used (formerly-public) function's documentation:
> On Windows, the taskbar button for the window flashes, if this hasn't been disabled by the user. > > On Linux, the behaviour varies from window manager to window manager - some flash the taskbar button, others focus the window immediately. This may be configurable as well. > > On Macintosh, the icon in the upper right corner of the desktop flashes.
