Variable font support can be used to fingerprint OS versions
In Tor Browser, the value of the layout.css.font-variations.enabled
preference changes depending on the operating system version. For example, it is enabled on Windows 10 and disabled on Windows 7 and this can be detected with javascript.
It can be tested here:
https://privacycheck.sec.lrz.de/active/fp_je/fp_js_echo.html
When the preference is enabled, the font-optical-sizing
and font-variation-settings
properties will appear under HTML Elements.