Many browsers identify themselves when they request a document. A CGI script will have this information available in the HTTP_USER_AGENT environment variable, and it can use that to send out a version of the document which is optimized for that browser. Keep in mind not all browsers identify themselves correctly. Microsoft Internet Explorer, for example, claims to be "Mozilla 1.2" to get at Netscape enhanced documents. And of course, if a cache proxy keeps the Netscape enhanced document, someone with an other browser will also get this document if he goes through the cache.