Without knowing this for sure I would imagine the distinction between setting an event handler and adding an event handler is made on purpose.

The description of MIDIOutput.open() is surprisingly close to the one for MessagePort.start(). Calling start() on a MessagePort is also not necessary when using onmessage. But it's necessary when adding an event handler with addEventListener().

Yes, but this is not what has been shipped for as long as Web MIDI has been shipped, and not doing it breaks lots of websites ("for Web Compat reasons").

Sorry, I misunderstood the issue. Because of the links I thought you meant onevent and addEventListener('event', ...) should generally do the same thing.

I didn't even know that start() gets called automatically in Chrome when using addEventListener(). I always did it manually. :-)

Audio Working Group 2023-10-05 meeting conclusions:

• We should add the requirement for addEventListener as well
• If possible, copy language directly from the spec links in the first comment on this issue

For the Web compat (Chromium) behavior, I think the implicit call is only on a MIDIInput, not on a MIDIOutput:
https://crsrc.org/c/third_party/blink/renderer/modules/webmidi/midi_input.cc?q=setOnmidimessage

This is also the behavior I see when trying it. Adding a listener or event handler to a MIDIOuptut does not seem to open the output port.

I will raise the PR specifying Input and ask Paul to confirm.