Gregory's Server@simon i agree that a "document" in a tab or a frame is a good sandbox. But I doubt very much one can achieve slfurther segregation within a document. there are way too many ways of linking back to javascript from html or svg tags, for example. And JS, on its side, has no segregation or protection whatsoever: one is free to manipulate all the DOM and beyond.
@gwidion it looks to me like https://claude.ai has a robust solution to this, using a combination of iframes with the sandbox attribute and CSP headers, plus web workers with CSP headers and careful application of postMessage
I'm still trying to reverse engineer how their solutions work though