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Font measurements are not a unique ID

Font measurements are not a unique ID?

Pillow on this machine drew ten M characters and ten i characters at two fonts, and the widths were 90 and 30 for the default font and 138 and 45 for DejaVu Sans at 16 pixels, which shows a font change can change a measured width.

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Pillow on this machine drew ten M characters and ten i characters at two fonts, and the widths were 90 and 30 for the default font and 138 and 45 for DejaVu Sans at 16 pixels, which shows a font change can change a measured width.

Last checked 2026-10-22

The two drawings

Pillow 10.2.0’s default font: MMMMMMMMMM was 90 pixels wide and iiiiiiiii was 30. DejaVu Sans at /usr/share/fonts/truetype/dejavu/DejaVuSans.ttf, size 16: 138 and 45. The version string is PIL.__version__ from the session run.

The gap is the point. The same ten letters are wider in DejaVu Sans 16 than in Pillow’s default font. A page that measures text can see a font difference. It cannot, from this table, say how many people share a width.

The browser button

The WebGL and client hints page measures the same two strings at 16 pixels in sans-serif, serif, and monospace, and only after a click, on a canvas that is not the WebGL canvas. In the headless SwiftShader Chrome on this machine, those widths were 133.28125 and 35.546875 for sans-serif, 142.265625 and 44.453125 for serif, and 96.328125 for both strings in monospace. That is one process, not a share of people.

Sources

Last checked 2026-10-22.

Questions

Is 90 a browser fingerprint?

No. Those widths are Pillow drawing on this machine, not a canvas measurement from a visitor’s browser, and they are not a percentage of people who share them.

What does document.fonts tell you?

MDN says document.fonts returns the document FontFaceSet. document.fonts.ready fulfills when loading and layout of used fonts are done. Used fonts can differ from declared fonts, optional fonts may not load in time, and some fonts stay unloaded if nothing uses them.

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