Ah! ServiceWorkers do have type module support. caniuse.com/mdn-api_serv... , that page was talking about background service workers which have restrictions in running remote code so no import() maybe.

Round 2 though, no, importmaps aren't supported so good luck have fun go-fly-a-kite trying to use modules in any reasonable honest way. Ffff. github.com/WICG/import-...

How do we install import maps in worker/worklet contexts? · Issue #2 · WICG/import-maps

It's unclear how to apply import maps to a worker. There are essentially three categories of approach I can think of: The worker-creator specifies the import map, e.g. with new Worker(url, { type: ...