WPT details: Fully tested. Web platform tests for module script loading in shared workers can be found in wpt/workers/modules directory. You can see the test result from wpt.fyi. Also we have web-platform-tests here as well: content-security-policy, referrer-policy, mixed-content, upgrade-insecure-request. SharedWorker cannot be nested nor called from Dedicated Workers, so there are no tests for constructing SharedWorker from workers.
JavaScript will support modules in shared workers. Setting 'module' type by the constructor's type attribute, worker scripts are loaded as ES modules and the import statement is available on worker contexts.
With this feature, web developers can more easily write programs in a composable way and share them among a page and workers.
Docs: https://docs.google.com/document/d/1sSdYdSOLd5zvnNGeVNlBqfTZs_VRPIrrigV3SlMnWjg/edit?usp=sharing
Explainers: https://html.spec.whatwg.org/multipage/workers.html#shared-workers-and-the-sharedworker-interface