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

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

Measuring text widths for font detection…

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