Layout and compositing need to be cross-origin; the Web implicitly allows cross-origin resource loads; and (as was raised above by pcwalton) browsing contexts l

Layout and compositing need to be cross-origin; the Web implicitly allows cross-origin resource loads; and (as was raised above by pcwalton) browsing contexts like windows and frames need to function as if they're running on the same thread and can potentially be shared. All of that is to say that all of the potentially shared state makes it extremely difficult to bind a web origin to a process for security purposes. It's a hard enough problem that a team of Chrome engineers has been working on it for a few years, and are only now approaching an alpha implementation.* And that's many years after Chrome was entirely multi-process with sandboxed web renderer processes. * http://www.chromium.org/developers/design-documents/site-isolation http://www.chromium.org/developers/design-documents/site-iso...