An Interactive Introduction to Fourier Transforms
At the time of writing the Gibbs phenomenon -- the overshoot occurring at discontinuities of a square wave, which does not go away no matter how many harmonics are included (see <a href="https://en.wikipedia.org/wiki/Gibbs_phenomenon" rel="nofollow">https://en.wikipedia.org/wiki/Gibbs_phenomenon</a>) -- is not correctly represented in these visualisations, implying that important aspects of the underlying maths are obscured. In my opinion this limits the didactic value of an otherwise impressive presentation.