This recent article contains a bit more detail on Site Isolation: https://arstechnica.com/gadgets/2017/12/chrome-63-offers-eve...
> Chrome's default model is, approximately, to use one process per tab. This more or less ensures that unrelated sites are kept in separate processes, but there are nuances to this set-up. Pages share a process if they are related through, for example, one opening another with JavaScript or iframes embedding (wherein one page is included as content within another page). Over the course of a single browsing session, one tab may be used to visit multiple different domains; they'll all potentially be opened within a single process. On top of this, if there are already too many Chrome processes running, Chrome will start opening new pages within existing processes, resulting in even unrelated pages sharing a process.
Which suggests there are a number of cases where multiple tabs could share a process.
[1] Two pages are considered cross-site if they cannot use document.domain to become same origin. In practice, this means that the effective TLD + 1 component match.
Curious to know whether Firefox has anything similar in the pipe, since it uses a fixed number of content processes rather than a variable number of processes.
