I'm using Worker DOM to run embedded 3rd party code.

If a third-party insert style like <style>* { color: red !important }</style> in the Worker, it can easily affect the host.

Although it was confirmed that encapsulation was possible simply by using an element attached through a shadow root instead of directly using the base element.

Wouldn't this be nice on upstream too? If that's okay, I'll file a PR.