I understand, but things like https://github.com/GoogleChromeLabs/comlink enable it. Similar to how iFrames don't have access to their parent page you need a facilitator. My question is why not use a js facilitator that could work in all browsers, rather than just Chrome.
I find it an interesting choice that the author decided to invest in new iFrame technology rather than existing multi-thread technology in the browser.
I don't think Comlink supports DOM as well. It's just syntax sugar over Web workers making them easier to use rather than providing new functionality over them.