How does canvas font detection work without asking permission?

The browser renders a test string in a candidate font with a baseline fallback. If the canvas width differs from the baseline's width, the candidate font must be installed — the browser used it.

Why are some fonts I have installed not detected?

The method has false negatives when a font's metrics closely match a baseline, and only checks a fixed candidate list. The newer queryLocalFonts() API is more accurate but needs a user permission prompt.

Is the list of installed fonts used for fingerprinting?

Yes — font set is one of the higher-entropy signals for browser fingerprinting. Privacy browsers like Tor Browser deliberately restrict or unify it.

Font Detector

Measuring text widths for font detection…

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Find out which fonts your browser has access to, rendered in their own typeface as a preview.