That's a bit of an open question at the moment. The obvious choice from a Rust ecosystem perspective (easiest to integrate) would be Boa (https://boajs.dev/). It has excellent standards conformance, but terrible performance. We'd need to test to what extent that would be an issue in practice.

Other engines on my radar: quickjs, hermes, primjs (and of course V8/JSC/SM, but they don't exactly fit with the "lightweight ethos").

There is also the possibility of bindings with more than one engine and/or using some kind of abstraction like NAPI or JSI.