Search code examples
user-agent

Why do all browsers' user agents start with "Mozilla/"?

All popular browsers' user agent strings, even Internet Explorer's, start with Mozilla/. Why is this the case?

Solution

  • 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'.