Editing User-Agent header (section)

== User agent sniffing ==
{{main article|Browser sniffing}}

''User agent sniffing'' is the practice of websites showing different or adjusted content when viewed with certain user agents. An example of this is [[Microsoft Exchange Server]] 2003's Outlook Web Access feature. When viewed with Internet Explorer 6 or newer, more functionality is displayed compared to the same page in any other browsers. User agent sniffing is considered poor practice, since it encourages browser-specific design and penalizes new browsers with unrecognized user agent identifications. Instead, the W3C recommends creating standard HTML markup,<ref name=www.w3.org>{{cite web|last=Pemberton|first=Stephen|title=W3C Markup Validation Service|url=http://www.w3.org/MarkUp/#guidelines|publisher=W3C|access-date=2011-10-18}}</ref>  allowing correct rendering in as many browsers as possible, and to test for specific browser features rather than particular browser versions or brands.<ref>{{cite web|title=Browser Detection and Cross Browser Support|url=https://developer.mozilla.org/en/Browser_Detection_and_Cross_Browser_Support|work=[[Mozilla Developer Center]]|publisher=[[Mozilla]]|first=Bob|last=Clary|date=10 February 2003|access-date=2009-05-30|archive-date=2011-11-17|archive-url=https://web.archive.org/web/20111117014255/https://developer.mozilla.org/en/Browser_Detection_and_Cross_Browser_Support|url-status=dead}}</ref>

Websites intended for display by mobile phones often rely on user agent sniffing, since [[mobile browser]]s often differ greatly from each other.