The substatus codes are not officially recognized by IANA and are not returned by non-Microsoft servers.Microsoft's IIS 7.0, IIS 7.5, and IIS 8.0 servers define the following HTTP substatus codes to indicate a more specific cause of a 404 error: found that "404" became a slang synonym for "clueless" in the United Kingdom.
The following two digits indicate the specific error encountered.
HTTP's use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP.
The webserver software developed by Microsoft, Microsoft's Internet Information Services (IIS), returns a set of substatus codes with its 404 responses.
The substatus codes take the form of decimal numbers appended to the 404 status code.
At the HTTP level, a 404 response code is followed by a human-readable "reason phrase".
The HTTP specification suggests the phrase "Not Found" and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase.Many organizations use 404 error pages as an opportunity to inject humor into what may otherwise be a serious website.For example, Metro UK shows a polar bear on a skateboard, and the web development agency Left Logic has a simple drawing program.In July 2004, the UK telecom provider BT Group deployed the Cleanfeed content blocking system, which returns a 404 error to any request for content identified as potentially illegal by the Internet Watch Foundation.In Tunisia, where censorship was severe before the 2011 revolution, people became aware of the nature of the fake 404 errors and created an imaginary character named "Ammar 404" who represents "the invisible censor".Some websites report a "not found" error by returning a standard web page with a "200 OK" response code, falsely reporting that the page loaded properly; this is known as a soft 404.