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?

The top right button has a "Download code" which gives you a .zip file. That .zip file doesn't have any JS in it, and renders the terrain just like in the online editor, except you can turn off JS and it still works.

Edit: someone else wrote basically the same an hour ago: https://news.ycombinator.com/item?id=45814791

	▲	thih9 2 days ago \| parent [-]
		Looks like it’s a “(css-only terrain) generator” - a generator that lets the user create and download a css only terrain. As opposed to a “css-only (terrain generator)” - a terrain creation studio built with css only.