Standard Mozilla behaviour last time this question came up is to include a list of fonts that your browser can display; I don't know whether other browsers do the same, or if they've changed it, but it's the kind of "feature" that hopelessly breaks your chances of non-uniqueness if you've ever installed fonts.
My work laptop has a font that's the Official Corporate-Branded font for $DAYJOB's corporate logo. Almost every Windows machine at my company has that (at least, every physical machine and the virtual machines running on the hosted virtual desktop cloud; there may be some lab machines that don't, and maybe some contractors, etc.) You might work for a smaller company that does the same. In my case, I've installed all sorts of other random fonts, either to see what they looked like, or simply because back in the 80s of course you wanted Elvish and Dwarvish fonts on your computer, or because I wanted a better monospaced programming font than the default MS one or Courier New.
Lots of other things leak information as well (cookies, etc.), but fonts are a quick and dirty way around identifying people who block those.