Each algorithm step does not say "parallel", "queue tasks", or anything in the specification linked to these will be executed synchronously. *

What’s in the specification algorithmdispatchThe target event is always executed synchronously. It comes from an EventTarget#dispatchEvent(), HTMLElement#click() or from an actual user initiated event.

Confusingly, for most cases, this scheduling event algorithm itself is wrapped in a queuing the task call , so actually the occurrence of the event will be asynchronous.

For example, when the image loads we have

Where "Queue Element Task" will call our Queue Task algorithm, and "Trigger Event" will call our > Scheduling Algorithm .

So, yes, in this case, the load event fires asynchronously after the image loading actually occurs.

Now, if we go back to EventTarget#dispatchEvent() (and HTMLElement#click( ), it boils down to a lot of events same build steps), we can see that it indeed calls the dispatch algorithm directly without queuing any new tasks.

Not only calls the algorithm synchronously, it even returns the result of the algorithm to the caller. Therefore the algorithm cannot be processed in parallel and the caller must call all handlers synchronously to get the result.

const makeEvent = (type) => new Event(type, { cancelable: true });
addEventListener("foo", (evt) => {
  console.log("foo fired");
});
console.log("return value", dispatchEvent(makeEvent("foo")));

addEventListener("bar", (evt) => {
  console.log("bar fired");
  evt.preventDefault(); // cancel the event
});
console.log("return value", dispatchEvent(makeEvent("bar")));