[nolist_policy]

The best isolation is inside a Service Worker, where the script is served with Content-Security-Policy: sandbox header.[1]

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/...

[Lerc]

I'm not sure if service workers are particularly amenable to having Developer A provide an interface for User B to run untrusted code made by Developer C, D and E.