Maybe I'm just used to 'tabs' with 4 spaces. It's what Douglas Crockford recommends (I'm not a fanboy, mind you, and I disagree with some of his opinions).

About the coding style though, you can take a look at the oldest version of the file:

https://github.com/cwilso/WebMIDIAPIShim/blob/0bc71383179d084f48238023d51b3edb2bfac230/WebMIDIAPI.js

You'll notice that the indentations are not exactly consistent (the 2-space and 4-space salad).