relatedNode is used to identify a secondary node related to a mutation event. For example, if a mutation event is dispatched to a node indicating that its parent has changed, the relatedNode is the changed parent. If an event is instead dispatched to a subtree indicating a node was changed within it, the relatedNode is the changed node. In the case of the DOMAttrModified event it indicates the Attr node which was modified, added, or removed.