A Visual Exploration of Gaussian Processes (2019)

What a stunning interactive visualization. I wonder what tool was used here?

Edit: Here's their template https://github.com/distillpub/template/tree/master

As I feared, they seem to be using D3.js "by hand", which is the only way to achieve such high quality but needs IMO an enormous amount of work and dedication. No wonder they are offline, that's hard to sustain. I hope a technology comes that makes it significantly easier to make such interactive charts. I know there are many out there, but there's always a "but".