[ndr]

related self plug: I built https://codeglf.com on Pyodide, a weekly Python code golf site where every submission runs locally in the browser.

WebAssembly's isolation means you get sandboxing for free without any server infrastructure. Of course users could cheat and submit code that doesn't actually pass the tests, but so far everybody has been friendly and I haven't needed to validate results server-side. Hopefully sharing the link here doesn't make that worse!