OK. Chrome can work with MIDI devices (and Electron apps as well – like Slack)
The example will work, sort of, by showing you the MIDI data from a device. I didn’t plug in a device, appeared not to work.
I tried this demo https://webaudiodemos.appspot.com/midi-synth/ to see if this worked. Noticed menu-like bit for MIDI-IN, chose a MIDI device, pressed notes, it worked.
Back to the original example. This now shows data.