Butterick made his own programming language called Pollen [1] to publish this site. [2] Pollen is implemented in the Racket programming language. [3] He has another online book called Beautiful Racket [4] about language-oriented programming in Racket.
[1] https://pollenpub.com
[2] https://practicaltypography.com/why-racket-why-lisp.html
[3] https://racket-lang.org
[4] https://beautifulracket.com