Apparently the site itself is written in Loon. The HTML is just a static shell that loads a `boot.js` script[1] that runs some WASM that compiles and evals the Loon source files. I found the source code here[2].

Definitely cool in concept, but very performance-intensive and slow.

[1]: https://loonlang.com/boot.js

[2]: https://github.com/ecto/loon/tree/main/web