[jackwilsdon]

Looks like it's using the WebAssembly build of the compiler which compiles to JavaScript:

https://github.com/gleam-lang/language-tour/blob/6d69cee90a3...

Although currently it's running v0.34.1 and not v1.0.0 (there doesn't seem to be a v1.0.0 wasm build yet): https://github.com/gleam-lang/language-tour/blob/6d69cee90a3...