I'm assuming it's the render engine that is in pure CSS. You could display a static map in CSS but things like the tools to modify the terrain definitely need JS.
I wanted to check if your assumption is correct but I couldn’t find the source code.
Why do you think the renderer is pure css and not e.g. mostly css?
GP linked an example of a similar project that allowed you to modify the terrain without any JS at all
You might not need it using the new :has() and different inputs as modifiers. Though that's a lot of :has() and probably would kill performance.