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http://w3c.github.io/webcomponents/spec/shadow/#event-retargeting The algorithm relies on ORIGINAL-TARGET to point to some node, since without that TARGET-TREE doesn't make sense. But ORIGINAL-TARGET is not defined anywhere.
Fixed in https://github.com/w3c/webcomponents/commit/58ef0217d0380cb3ef4248dd7039462b2c9d9ff1