Spec: https://html.spec.whatwg.org/multipage/browsers.html#the-history-interface, https://html.spec.whatwg.org/multipage/browsers.html#dom-history-back,
Code: https://github.com/servo/servo/wiki/Adding-a-new-WebIDL-binding

This will just end up mimicing the behaviour of the GoForward/GoBack APIs of HTMLIFrameElement (see components/script/dom/element/htmliframelement.rs)