In section "6.6.6 Changes to the networking model" which applies to sub resource loads, step 3 prevents returning fallback resources for requested urls that fall into a network namespace.
In section "6.5.1 Navigating across documents" which applies to main resource loads, step 17 does not explicitly exclude returning fallbacks for such urls. I doubt this difference is intentional, looks like step 17 needs some <additional words>... "If the resource was not fetched from an application cache, and was to be fetched using HTTP GET or equivalent, and its URL matches the fallback namespace <but not the network namespace> of one or more relevant application caches..."
