The opener IDL attribute on the Window object on getting must return the WindowProxy object of the browsing context from which the current browsing context was created its opener browsing context if there is One if it is still available and if the current browsing context has not disowned its opener otherwise it must return null. On setting if the new value is null then the current browsing context must disown its opener if the new value is anything else then the user agent must call the DefineOwnProperty internal method of the Window object passing the property name opener as the property key and the Property Descriptor Value value Writable true Enumerable true Configurable true as the property descriptor where value is the new value....browsing context), if there is one, if it is still available, and...