The answer is: It claims to be, but it proably isn’t.
If you surf to this site with Internet Explorer (the user experience may not be so good as the layout of this site is fairly broken in Microsofts flagship browser), your browser will identify itsself with our server sending a special token called user agent string.
For the new Internet Explorer 7.0 on the new Windows Vista, this token looks like this:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)
In its MSDN library article Understanding User-Agent Strings Microsoft writes:
For historical reasons, Internet Explorer identifies itself as a Mozilla 4.0 browser.
and
The Compatibility flag (“compatible”) is used by most modern browsers; it indicates that Internet Explorer is compatible with a common set of features.
This is of course right. However, they could have provided a bit more information.
If the part Mozilla/4.0 reminds you of Internet Explorers rival Mozilla Firefox (highly recommended), this is not without cause. Back in the early days, when the Netscape browser still dominated the internet, Mozilla was the code name of Netscapes core rendering engine used to display the web pages on your screen. When Netscape open sourced their code in 1998, they named the new open source project after the code name of their rendering engine. The project was called Mozilla.org. And this is also the home of the now famous Firefox browser.
Because of the so-called browser sniffing, Internet Explorer was one of the first browsers to implement user agent spoofing. Wikipedia gives us all the information about the origin of the word Mozilla in Internet Explorers user agent string:
The earliest example of this is Internet Explorer’s use of a User-Agent string beginning “Mozilla/
(compatible; MSIE …”, in order to receive content intended for Netscape Navigator, its main rival at the time of its development. It should be stressed that this is not a reference to the open-source Mozilla browser, which was developed much later, but to the original codename for Navigator, which was also the name of the Netscape company mascot. This format of User-Agent string has since been copied by other user agents, partly because Explorer, in turn, came to dominate.
This means Internet Explorer 7.0 identifies itsself as Mozilla/4.0, which refers to the rendering engine of Netscape 4. The detailed information (in parenthesis) contains the string compatible. This means Internet Explorer 7.0 claims to be compatible with the rendering engine of Netscape 4 and it also shows to anyone, who’s interested, that the visiting browser is not actually Netscape 4, but just a compatible bowser. The compatible set of features Microsoft mentions in their article are that of Mozilla/4.0, the rendering engine of Netscape 4. The rest of the parenthesis part refers to the actual name and version of the browser and the operating system of the client computer.
For the record, this is the user agent string of Mozilla Firefox 1.5.7 on Debian Linux:
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.0.7) Gecko/20060909 Firefox/1.5.0.7
Of course we wouldn’t have expected from Microsoft to mention their old (and somehow new) rival in the browser market (Netscape and Mozilla) in one of their MSDN articles…
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