XML in a Nutshell, 3rd Edition

Content Negotiation

HTTP clients and servers support a variety of accept headers that indicate which kinds of content the client is prepared to receive. For example, this browser request indicates that the client prefers French but is willing to read English; can handle HTML, plain text, and JPEG images; knows how to decode gzipped data; and recognizes the ASCII, Latin-1, and UTF-8 character sets:

GET /index.html HTTP/1.1
User-Agent: Mozilla/4.6 [en] (WinNT; I)
Host: www.cafeaulait.org
Accept: text/html, text/plain, image/jpeg
Accept-Encoding: gzip
Accept-Language: fr, en
Accept-Charset: us-ascii, iso-8859-1,utf-8
Connection: close
If-Modified-Since: Sun, 31 Oct 1999 19:22:07 GMT

The server that receives this request uses these headers to decide which version of a resource to send to the client. The same URL can return different content depending on how these headers are set. In browsers, this is normally controlled through preferences, but XInclude allows documents to control two of these headers, Accept and Accept-language, by attributes. Each xi:include element can have an accept and/or accept-language attribute. The values of these attributes should be legal values for the corresponding HTTP header fields. If one or both of these attributes is present, then the XInclude processor will add the relevant accept headers to the HTTP request it sends to the server. For example, this xi:include element indicates you want to include the French HTML version of Google’s home page:

<xi:include ...