URI共通構文（F）

   The URL syntax was designed for unambiguous reference to network
   resources and extensibility via the URL scheme.  However, as URL
   identification and usage have become commonplace, traditional media
   (television, radio, newspapers, billboards, etc.) have increasingly
   used abbreviated URL references.  That is, a reference consisting of
   only the authority and path portions of the identified resource, such
   as

      www.w3.org/Addressing/

   or simply the DNS hostname on its own.  Such references are primarily
   intended for human interpretation rather than machine, with the
   assumption that context-based heuristics are sufficient to complete
   the URL (e.g., most hostnames beginning with "www" are likely to have
   a URL prefix of "http://").  Although there is no standard set of
   heuristics for disambiguating abbreviated URL references, many client
   implementations allow them to be entered by the user and
   heuristically resolved.  It should be noted that such heuristics may
   change over time, particularly when new URL schemes are introduced.

   Since an abbreviated URL has the same syntax as a relative URL path,
   abbreviated URL references cannot be used in contexts where relative
   URLs are expected.  This limits the use of abbreviated URLs to places
   where there is no defined base URL, such as dialog boxes and off-line
   advertisements.