HTTP/1.0 cache control
HTTP/1.0 uses the Pragma header set to the value no-cache to prevent caching. Some old user agents don't support the use of the Pragma header to control the caching of a response. The only way to prevent caching of a page with these older user agents is to set the Expires header field to 0, which instructs many user agents to immediately expire the response or change it to a past date and time. This practice is recognized, but not formally supported by the HTTP standard.