The easiest way for site-owners to delegate control has been to include third-party javascript. With new browser restrictions, we're starting to see companies switching to loading JS via CNAMEd subdomains, because that's nearly as easy. The next step is probably reverse proxies, though, where the third-party JS comes from the same server that gives you the rest of the site's JS.
(Disclosure: I work in ads; speaking only for myself)
(Disclosure: I work in ads; speaking only for myself)