fonts

fonts at a glance (from the library code, signalsVersion 5)

Component name	fonts
In the visitorId	yes
structuredId group	fonts
Hashing	always a digest, except the plain value none when nothing is found
Excluded under	Brave (brave_farbling)
Inner timeout	1000 ms
Added in	signalsVersion 1

What it reads

The 54 families are a fixed list of fonts common on Windows, macOS, Linux, Android and iOS, for example Arial Black, Calibri, Consolas, Helvetica Neue, Menlo, Noto Color Emoji, Roboto, Segoe UI, SimSun, Ubuntu and Yu Gothic. For each family WhiskerJS creates three <span> elements with the sample text mmmmmmmmmmlliWWQ@#0OfiflK at 64 px, set to '<family>', monospace, '<family>', sans-serif and '<family>', serif, plus one span per plain fallback. All spans sit in one container placed at left: -9999px with visibility: hidden, appended to document.body, measured, and removed in a finally block.

How the technique works

1. Measure the width and height (offsetWidth, offsetHeight) of the text in each generic fallback alone.
2. Measure the same text in each family with each fallback behind it.
3. If the family is missing, the browser uses the fallback and the size is identical. If it is installed, at least one width or height differs, and the family counts as present.
4. The found families are sorted and joined; the list is hashed. When none is found, the component reports the plain word none instead of a digest.

All spans are created first and measured afterwards, so the browser lays out the container once.

Why it helps

Installed fonts vary a lot: an office suite, design software or a language pack each add families, and each operating system ships its own set. The list of found families is one of the more distinguishing values in the fonts group.

Stability

It changes when the user installs or removes one of the 54 fonts, when an operating system update adds or removes one, or when a browser restricts which system fonts pages can use. Page zoom does not change which fonts are found.

Browser support and what blocks it

• ok in every browser with a document and layout.
• Brave: excluded / brave_farbling, because Brave randomises the set of fonts visible to pages.
• Browsers or privacy settings that limit pages to a small allowlist of fonts make the found list short; it is still read and reported. Firefox resistFingerprinting is not excluded for this component.
• unavailable / no_document without document.body (for example when collecting before the body exists), no_layout when nothing is laid out.

Privacy

It reveals which of 54 well-known fonts are present, which can hint at the operating system, languages or installed software. Only a digest of the found list leaves the library. The spans are invisible, exist only for the measurement and are removed at once. No permission, no prompt, no request.

How WhiskerJS handles noise and failures

Status	ruleId	When
ok	none	a digest of the found list, or plain none
unavailable	no_document	no document, no document.body
unavailable	no_layout	the fallback spans had no width
unavailable	no_digest	no Web Crypto
blocked	security_error, threw	the DOM work threw
excluded	brave_farbling, caller_excluded	exclusion table or your list
timed_out	timeout, aborted	1000 ms inner timeout, budget, abort

Example value

From a headless Chromium test browser in a container:

"fonts": { "status": "ok", "value": "e2727c20c7bf4d2b9aa270f3427907a0", "hashed": true }

Questions and answers

Related pages