Those first two interactive diagrams leave something to be desired. The harmonics are shown out of phase with the fundamental, which won't be the case for real harmonics of a note. If you are going to demonstrate something, get it right!

To better illustrate the point, I'd suggest showing the fundamental and the harmonic waveform, as well as the resultant waveform. It makes it easier to see how the resulting wave is formed. Also showing the effect of the amplitude of the harmonics on the resulting waveform would also have been useful before moving on to more complicated items.

Not impressed with that site at all.