Why do all browsers’ user agents start with “Mozilla/”?

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It is a long and sad story.

In summary:

  1. Mozilla browser gets released, with User-Agent Mozilla/1.0 (Win3.1). It is publicly renamed to Netscape, but in its User-Agent it keeps its original name .
  2. Internet Explorer is released. It spoofs Netscape by starting its User-Agent with Mozilla/ because web servers were routinely browser sniffing and serving pages with frames – a feature supported by both Netscape and IE, but not other browsers of the era – to Netscape only.
  3. Over time, Gecko, Konqueror, Opera, Safari and Chrome each decide to similarly spoof the User-Agent of some previous browser in order to manipulate browser-sniffing web pages into correctly understanding their browser’s features. As part of this spoofing, all the browsers start their User-Agents with Mozilla/, like the browsers before them had done. Plenty of other nonsense also results, like modern Chrome’s User-Agent simultaneously claiming to be Mozilla, Chrome, Safari, and ‘like Gecko’.

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