Web MIDI isn't for playing .mid files in the browser. It's for letting in-browser JavaScript talk to arbitrary hardware devices that speak the MIDI protocol; th

Web MIDI isn't for playing .mid files in the browser. It's for letting in-browser JavaScript talk to arbitrary hardware devices that speak the MIDI protocol; this includes not only things like keyboards and synthesizers, but also stage lighting systems and other theatrical devices that use MIDI as a control protocol (so that you can have, e.g., lighting and sound cues happen in sync, because they're all centrally controlled by a single computer). In general, any API that controls hardware counts as security-sensitive; you don't want MITMs to be able to hijack your hardware devices and make them do something bad.