There is a mechanism called Freshness check that is done periodically by the caching device inorder to ensure that the content it is serving is the latest. So if you cache a copy and shut down or disable the origin server, there is no way the cache can validate the content and hence it does not serve it to the client and the web page cannot be opened. Pragma is a browser property, When present in a request header it indicates that all intermediate proxy servers should forward the request all the way to the origin server even if a fresh copy exists in cache. So this wold also require access to the Origin server.
