As an update, I am planning on working on this within the next few weeks... it ended up being implemented in servo/servo@29d1cf6, more or less by re-using the concept of the 'event-path'

Interesting. Why not iterate over the listeners of the EventTarget object's event listener list?

I'm not sure, I think I assumed the entire path had to be checked...

So essentially you mean, taking the linked to implementation as a context, just do target.has_listeners_for(type_) on the one EventTarget and not bother with doing that for all others in the event path for that target?

Yeah, I think that's what https://searchfox.org/mozilla-central/rev/ae8c2e2354db652950fe0ec16983360c21857f2a/dom/events/EventListenerManager.cpp#1609-1620 and its callers do so that would match what implementations do.

Thanks, I'll investigate further and update the Servo implementation.

whatwg/html#5889 removed "if any event listeners were triggered" from HTML, so this seems to reduce to #660.

@jakearchibald @jungkees so I guess only service workers still needs this hook? Does #660 work for you?

XMLHttpRequest also needs this

If one or more event listeners are registered on this’s upload object, then set this’s upload listener flag.

and has a test at xhr/event-upload-progress.any.js.