This issue is part of an effort to improve Servo's embedding story. See https://github.com/paulrouget/servoshell/projects/2

We want the embedder to have the ability to prevent navigation. For example, if the embedder wants to open "http://reddit.com" link in a different browser, it needs to be able to cancel the navigation and handle the URL itself.

WindowMethods::allow_navigate(&self, url: ServoUrl) -> bool;