!!! Overview[1]
[{$pagename}] is a mechanism defined in the [HTTP] specification that makes it possible to serve different __versions__ of a document (or more generally, a resource representation) at the same [URI], so that user agents can specify which version fit their capabilities the best. 

The server is then able to supply the version of the resource that best fits the user agent's needs.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Content negotiation|Wikipedia:Content_negotiation/|target='_blank'] - based on information obtained 2016-06-25-