The AlertPopupInterface QML type is exposed through the Sailfish.WebView.Popups QML import namespace.
Clients who use a WebView in their application may wish to customize the appearance or behavior of the alert popup in their application.
To do so, their implementation must extend the AlertPopupInterface, which itself extends the UserPromptInterface, and then they must tell the WebView to use their implementation via the popupComponents property.
Note that a custom implementation of an AlertPopupInterface need not offer separate accept and reject actions; simply providing an accept action is sufficient (e.g. an "OK" button to emit accepted when triggered).