The HTTP/1.1 caches treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
No new Pragma directives will be defined in HTTP.
However, to guarantee a fresh version of the resource, many clients send three HTTP caching headers at once that works across all servers and proxies.
Unlike the Cache-control header, Pragma is only applicable to client requests, while Cache-Control can be used along the whole request-response chain.
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