What it is
A page cannot list your fonts, but it can ask for one by name and measure what it got. If the text is the same width as the generic fallback, the font is not installed; if it differs, it is. Repeat for a few hundred names and you have the installed set.
How websites read it
span.style.fontFamily = '"Segoe UI", sans-serif';
const installed = span.offsetWidth !== widthOf('sans-serif'); // fallback changed the width?The probe takes a few milliseconds per font and happens in an invisible element. The Lab tries about fifty families that mark Windows, macOS and Linux.
Why it identifies you
Office suites, design tools, language packs and personal taste all install fonts, so the set is both OS-specific and personal. It is one of the higher-entropy signals a desktop browser exposes and it changes slowly.
How risk-control systems use it
The set is checked against the claimed OS: Segoe UI, Calibri and Consolas ship with Windows; Helvetica Neue, Menlo and Geneva with macOS. Missing the core fonts of your own OS is a soft flag; having the complete marker set of another OS is a hard one.
Common mistakes
A Windows profile on a Mac host with the host’s fonts showing through; a font list of three entries; the same handful of fonts on every profile; fonts from three operating systems at once.
Mango is accepting waitlist registrations. These capabilities describe its first release; client access opens with invitations.
How Mango Browser handles it
Custom10–16 fonts drawn from the OS-specific pool so the list always contains the OS core fonts and nothing from the wrong platform.
Check yours
Jump to this signal in your scan results.