This is only tested and known to be functional in recent versions (as of March, 2016) of the following browsers. If you are using a different browser and having problems, you might want to try again in one of these.
Playing notes using the computer keyboard may not work in some browsers when the focus is on certain controls. If using the computer keyboard to play notes is not working, you should be able to make it work again by clicking a note on the piano keyboard or by clicking on the volume control slider.
Chords can be played by holding down multiple keys on your computer keyboard at the same time. For instance, hold down the A, D, and G keys (for notes C, E, and G) all at the same time to play a C Major chord.
Due to limitations of computer keyboards' abilities to distinguish some combinations of simultaneously-held keys, some complex chords cannot be successfully played using a computer keyboard.