It's a javascript-based imitation, much like all of those js-based imitations of various Windows versions.
The original source code isn't really involved, which is a shame, since it is actually available.
IMHO this should have been (something along the lines of) GNUstep + TimBL's original code (mirror: https://github.com/cynthia/WorldWideWeb) + Emscripten + getting Emscripten to work with ObjC. Now, that would have been cool.
This is the most commented HN posting on this from that time (2019):
You can already run nextstep in browser, see https://aresluna.org/frame-of-preference/ (section "Back to 1992")
A WASM emulator of 68040 and NeXT, the original OS and compiler, then run WWW on top of that.
The performance would likely be comparable %)