[whatwg] Window events that bubble?

[David Flanagan]

Section 6.5.9 "History Traversal" defines popstate and hashchange events 
 that are fired on the Window object. It specifies that these events 
*must* bubble.  Where should they bubble to?  What does it mean to 
bubble up from a Window?  These events aren't going across frames, are they?

Is the specification that they must bubble a formality because existing 
implementations set the bubbles property of the Event object to true? 
Or does it actually have some impact on event propagation?

Thanks,

        David