Web Notifications defines an HTML5 API for end-user notifications. A notification allows alerting the user outside the context of a web page of an occurrence, such as the delivery of email.
This is an opt-in feature that usually needs to be enabled by the user per site. Common ways to show such notifications is via a system tray (see attached pictures for examples).
Qt does not offer API for showing notifications e.g. in the system tray. But Qt WebEngine should provide API to a) enable/disable notifications per page, and b) notify the application about arriving notifications.