Spec: https://html.spec.whatwg.org/multipage/webstorage.html#the-storageevent-interface, https://html.spec.whatwg.org/multipage/webstorage.html#send-a-storage-notification, https://html.spec.whatwg.org/multipage/webstorage.html#the-sessionstorage-attribute:send-a-storage-notification, https://html.spec.whatwg.org/multipage/webstorage.html#the-localstorage-attribute:send-a-storage-notification
Code: http://mxr.mozilla.org/servo/source/components/script/dom/storage.rs

The bit about "for every Document object whose Window object's localStorage attribute's Storage object is associated with the same storage area, other than x, send a storage notification" is the important bit to figure out here.