Font Fingerprint Test

By measuring rather than asking. The page renders a short string in the font it wants to test with a generic fallback behind it, then compares the width against the fallback alone. Different width means the font resolved and is installed. Nothing is displayed, no permission is requested, and a few hundred families can be checked in a few milliseconds.

Because it accumulates. On top of the families your operating system ships with, the list records what an office suite, a design tool, a language pack or a game installer added over the years, and that combination is rarely the same on two machines. It is also stable — fonts are not cleared with cookies and do not change between private windows — which is what makes it valuable to a tracker rather than merely interesting.

Partly. Some browsers and privacy extensions restrict resolution to a small standard set, which does cut the entropy. The trade-off is that the restriction is measurable with the same probe, and a machine that answers to almost nothing is unusual in a way an ordinary font list is not. It also breaks legitimate typography on sites that rely on locally installed families.

Both. The families available come from the system, but browsers differ in what they let a page resolve and how they handle fallbacks, so the same machine can produce slightly different lists in different browsers. That is expected. What should not differ is which operating system the set points to.

It means one of the two is not describing the machine underneath, and the font list is the harder one to fake. This is the classic outcome of changing a user agent string or running a profile configured for a platform other than the host: the string says one operating system, the installed families say another, and the pair contradict each other on the first page load. A believable identity needs the fonts, the string, the Client Hints and the GPU backend to agree.

Because the families that resolved do not point clearly at one platform. That happens on a machine with few fonts installed, where font access is restricted, or where the detected set is dominated by families that exist everywhere. It is an honest “cannot tell” rather than a failure — this check will not manufacture a confident platform guess out of Arial and Verdana.

We cannot say. Answering that would require a database of other visitors' font lists, which we do not keep and will not fabricate a number from. What this page gives you is the set this browser exposes, a stable digest of it so you can tell one run from another, and whether it agrees with the platform your browser claims.