There are ways to override it using "UserJS", in the case of the new Opera you can add custom JS into pages using an extension like ViolentMonkey.
See for example:
http://superuser.com/questions/705307/how-can-i-disable-are-you-sure-you-want-to-leave-this-page-popups-in-chrome
http://superuser.com/questions/593116/is-there-a-chrome-or-other-browser-plugin-that-blocks-javascript-alerts-that-f