Browser fingerprinting is an unintended side-effect of things it's sorta-kinda reasonable for browsers to provide.
A user agent that says the browser's version? Reasonable enough.
Being able to ask for fonts, if the system has them? Difficult to have font support without that.
Getting the user's timezone, language and keyboard layout? Reasonable.
The size of the screen, and the size of the browser window? Difficult to lay things out without that.
Of course a video or audio player needs to know which video formats your browser supports - how else to provide the right video?
Obviously javascript can get the time, and it's trivial to figure out the system's clock error by comparing that to the time on a server.
Before you know it, almost every browser is uniquely identifiable.
A user agent that says the browser's version? Reasonable enough.
Being able to ask for fonts, if the system has them? Difficult to have font support without that.
Getting the user's timezone, language and keyboard layout? Reasonable.
The size of the screen, and the size of the browser window? Difficult to lay things out without that.
Of course a video or audio player needs to know which video formats your browser supports - how else to provide the right video?
Obviously javascript can get the time, and it's trivial to figure out the system's clock error by comparing that to the time on a server.
Before you know it, almost every browser is uniquely identifiable.