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):
TimBL's original NeXT is still on display at CERN, I've seen it.
I've even stood in the office that was his when he wrote it (it was empty when I was there, but had recently been in use by some incredibly high-end physicist).
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):
https://news.ycombinator.com/item?id=19249373