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The 'error' progress event (http://xhr.spec.whatwg.org/#event-xhr-error) is never fired in the specification. However, a progress event named 'event' is fired several times in these steps: http://xhr.spec.whatwg.org/#request-error I think this may be a typo and that event should be named 'error' not 'event'.
At this point in the algorithm, the word "event" (in italics) is a variable that might be "error", "timeout" etc. Hence it does indeed sometimes indicate that the error event fires.
Oh right, my bad. I understand now. Thanks for the explanation.