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Firefox is experimenting with a font whitelist as part of upstreaming of Tor features into Firefox trunk. These features are controlled by the privacy.resistFingerprinting pref. resistFingerprinting has some webcompat issues, but Mozilla is considering a subset of the protections for Privacy Browsing mode, similar to Tracking Protection. Here is the work-in-progress bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1336208

I wonder if the fontSettings section of the Chrome extensions API[1] can be used to defend against that. By randomly and deliberately poisoning that info for when in Incognito mode (or even regular browsing).

[1] https://developer.chrome.com/extensions/fontSettings

Why does that feature even exist?

So dessigners can make adjustments depending on what fonts are available.

Is there any website actually using it for that purpose? Usual CI policies would be "actively disinterested" in it.

Any website with a rich text editor would use it.

AFAIK it's not really a feature it's more of an exploit.

Ultimately, the sizing of a font can effect element widths, and element widths can be queried.