It seems like the equation processing gets messed late in section 4 - $$ \mathrm{DFT}[k] = \sum_{n=0}^{N-1} \mathrm{x}[n] \cdot (cos(\varphi) - sin(\varphi)i) \\ where \quad \varphi = k \frac{n}{N} 2\pi $$ https://jackschaedler.github.io/circles-sines-signals/dft_wa... there are a few other examples in section 5 Aside from that, this is really great.
That's because you're accessing the site from HTTPS, or you have HTTPS Everywhere installed which is doing it for you. I think he's loading MathJax as HTTP, which means it gets blocked because it's coming from an insecure connection.
Thanks for this observation; I've commented on the existing GitHub issue to add this information.
https://github.com/jackschaedler/circles-sines-signals/issue...