Comment by boomlinde

The point is not to identify the people using Web MIDI but to identify individual users, regardless of what information exactly identifies them. To that end, every single piece of entropy helps. A good approach to it in general is to opportunistically consume every available API that can possibly divulge identifying information.

A lot of people also do have a virtual MIDI device installed whether they know about it or not. The name of this device differs between different operating systems and operating system revisions.