From Marcos:

It would be better if you could fold everything into MIDIPort and get rid of MIDIOutput and MIDIInput? you already have the port type, and you can just say that sending() does nothing when a port is not outputting.

If you don't agree, then I think MIDIInput and MIDIOutput need to inherit from MIDIPort (not implement the interface). Implementing the interface makes a huge mess when actually implementing, as the stuff from MIDIPort has to be copied over from MIDIPort.

So, worst case, please change the spec to match the following pattern:

interface MIDIOutput : MIDIPort {
}
interface MIDIInput : MIDIPort {
}
MIDIPort : EventTarget{
}

However, I strongly urge you to do away with MIDIInput and MIDIOutput. They are redundant, IMHO.