There will always be some overlap, but that doesn't entirely diminish the value of focus. It may be possible for a given window or panel to select which focus-standard it uses to make mix-and-match a bit easier.

A given "starting" browser, typically the document-oriented one which will contain menus and lists (such as an intranet) could be configured to select the user's or org's desired GUI and Art engine. And each "engine" will contain a minimal common sub-set, mostly around existing HTML.