 
                Vary Header check for User-Agent 
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                When using the Vary: User-Agent header, caching servers should consider the user agent when deciding whether to serve the page from cache
             
               
                
                
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                Vary Header Accept-Encoding
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                The Accept-Encoding request HTTP header advertises which content encoding, usually a compression algorithm, the client is able to understand. Using content negotiation, the server selects one of the proposals, uses it and informs the client of its choice with the Content-Encoding response header.
             
               
                
                
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                XFN 1.1
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                XFN is a technical term that is an acronym that stands for “XHTML Friends Network.” This is primarily used by websites that want to create a series of referral links between other websites in order to boost SEO ratings.