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"Let PATH be the result of running the event path calculation algorithm with HIT as input" The event path algorithm takes an EVENT argument. elementFromPoint has no EVENT object to pass.
The reason it takes an event object is that it needs to know if the event type should be stopped.
Thanks. Fixed at https://github.com/w3c/webcomponents/commit/00b36a0669e911cbdc457a369f9154c38060ce93