I’ll be working with a couple of guys on a 4-day hackathon cracking in to the Web MIDI API, and his work is shining a lot of light on this API that is currently sparsely documented.
If you’re looking for a starting point on learning and experimenting with the Web MIDI API, this is it.
Or if you really want to get crazy, read the W3C Working Draft. In all seriousness though, there are a few good, concrete examples in section 9 of the draft. You might want to skip sections 1-8 until you’re ready to dig into particulars of the spec.