The author, Christian Heilmann, has a geeky sense of humor that keeps the reading light — for eaxmple Et Tu, Cache? (pg. 309):
Safari is the main offender as it caches the response status and does not trigger the changes (remember that the status returns the HTTP code 200, 304 or 404) any longer.
Adding this snippet tells the browser to test whether the data has changed since a certain date, i.e.:
request.setRequestHeader( ‘If-Modified-Since’, ‘Thu, 06 Apr 2006 00:00:00 GMT’);
request.send( null );