As per this question, websites can learn what operating system a client is running by examining the HTTP headers. There are some addons that prevent this from working. What is a legitimate use of this, aside from targeted marketing?

  • It could be used to serve the right website (css, js, fonts etc...).
    – HamZa
    Commented Jan 26, 2016 at 10:39
  • @HamZa could you explain what you are thinking?
    – Celeritas
    Commented Jan 26, 2016 at 11:12
  • 3
    For example, I would serve a different site to an Android device than a Windows or OS X device. This could even vary between the versions of the same OS, for example css-tricks.com/os-specific-fonts-css
    – HamZa
    Commented Jan 26, 2016 at 11:15

1 Answer 1


One obvious answer is when the website contains applications that you can download - it can then offer the content appropriate for your operating system. If I go to www.videolan.org with Javascript turned off, the download link goes to the Windows binary, but if I turn Javascript on, it goes to the appropriate binary for the system I'm actually using.

  • wait just with js off they dont get Your OS anymore? I thought the OS is also included in the UserAgent...
    – My1
    Commented Jan 26, 2016 at 9:19
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    Another reason is for device fingerprinting
    – Purefan
    Commented Jan 26, 2016 at 9:20
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    @Purefan well ya, that's what I mean a legitimate use.
    – Celeritas
    Commented Jan 26, 2016 at 11:13
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    @My1: UserAgent header is a tragic piece of history, and getting any sort of useful data out is a pain. Using JS is significantly simpler. Commented Jan 26, 2016 at 12:37
  • but especially on mobile using UA is easier, becsause you dont even want to know how often I saw sites (especially forums and stuff) where I couldnt even use the js-powered submit button just because the js failed to load over mobile internet...
    – My1
    Commented Jan 26, 2016 at 12:48

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