codesandbox.io/ is pretty remarkable.
Free software, self-hostable, can show you a live rendering of a site or component as you're typing, supports live collaboration.
It's what's used for the #GitLab Web IDE.
It's a JS thing, but it supports backend in node, node has wasm support, and python and other languages have packages on wapm ... somebody could potentially do something unholy here to support arbitrary languages.
#codesandbox