[andybak]

I'm not sure it's meant to teach music theory. As you observe, that would get complicated.

> But then when you click the Cmaj button it only plays CGE.

Because those are the notes in Cmaj

> Why only three notes? A chord can have more than three.

Yes but not Cmaj.

I don't know if you're asking these questions yourself or putting yourself in the place of someone who is learning theory - but it seems a strange criticism of a single page interactive website that it fails to explain the vast amount of arbitrary information embedded in western standard practice.