Since Google Chrome v55 (and some previous versions) support for OOPIF (multi-threading iframes) is enabled by default. This provides many of the benefits of a WebWorker without startup latency and with full access to DOM.
https://www.chromium.org/developers/design-documents/oop-iframes
https://github.com/optimalisatie/exec.js/tree/master/tests (OOPIF vs WebWorker)
Does Firefox, Internet Explorer, Opera and/or Safari offer (or are planning to offer) something similar?
I've read about GeckoView and webview. It appears that Firefox has been working on it for mobile.