the CSS feature surface is identical in a fresh realm
Check id css-surface-vs-realm
What an ordinary browser yields
All fifty-odd answers identical in both realms — the surface is a property of the binary, and both realms run the same binary.
What a detector infers
Which CSS properties, values and selectors an engine understands is decided when the browser is compiled. The check asks CSS.supports() around fifty questions — modern layout (grid, container queries, :has()), engine-owned prefixes (-webkit-app-region, -moz-orient), and recent additions (anchor positioning, view transitions, field-sizing) — then asks the identical fifty inside a fresh same-origin iframe and compares every answer. There is no legitimate way for one realm of a browser to support a CSS property another realm does not: both are served by the same compiled style engine. So a disagreement does not mean the browser is unusual, it means the answers are being generated rather than reported, by something that reaches the page world but not a realm the browser builds fresh from its own binary. Note what this deliberately does not do: it never compares the feature set against a table of which Chrome version shipped what. Such tables are exactly the kind of hand-maintained corpus that goes stale and then starts accusing ordinary browsers of being fake — a failure this audit has already made once. Comparing a realm against another realm cannot rot, because both sides are measured live. This is the biggest single family in Kasada's deployed script after property enumeration: roughly forty-nine of its four hundred and twenty-seven probes are individual CSS.supports questions, asked in both realms.
How to resolve it
Do not rewrite CSS.supports or the style surface from page script. A fresh realm is constructed straight from the browser binary and will keep answering honestly, so patching the page world converts a capability into a contradiction. If a different feature set is genuinely required, it has to come from a different build.
Read in the wild by
Kasada
Roughly 49 of its 427 probes are individual CSS.supports questions, asked in both realms.
css_align, css_grid, css_shape, css_mask, css_moz, css_webkit, css_epub, … (49 total)
Kasada ips.js teardown — the full 427-probe listEvery attribution traces to a published artifact. See the sources and their limits.
Other checks in Capability surfaces
- the device-API family is present or absent as a unit
device-api-family-coherenceWeb Bluetooth, WebUSB, WebSerial and WebHID are one family on Chromium desktop - the same secure-context gating, the same desktop platforms… - Firefox resistFingerprinting announces itself when enabled
firefox-resist-fingerprinting-identifiableFirefox's privacy.resistFingerprinting substitutes CONSTANTS rather than randomness, and a constant is itself a fingerprint. - device-level media features are identical in a fresh realm
media-surface-vs-realmMedia features are answered by the layout engine rather than by the JavaScript properties that describe the same machine, which is what… - the codec matrix is identical in a fresh realm
codec-surface-vs-realmcanPlayType answers come from the media stack compiled into the browser, and the shape of that matrix is genuinely informative: branded… - supported <input> types are identical in a fresh realm
input-surface-vs-realmAssigning an unrecognised type to an <input> element does not throw — the element silently falls back to reporting type "text". - emoji advance widths are identical in a fresh realm
emoji-surface-vs-realmEmoji do not render from the page's fonts but from a platform font supplied by the operating system — Segoe UI Emoji on Windows, Apple Color… - the Web Animations surface is identical in a fresh realm
animation-surface-vs-realmThe Web Animations API exposes a small, precisely specified surface: whether Element.animate exists, whether the timeline and KeyframeEffect… - the platform theme resolves the CSS system colours coherently
system-colors-vs-platformThe CSS system-colour keywords — Highlight, HighlightText, Canvas, CanvasText, ButtonFace, ButtonText, GrayText and their newer siblings…
Clearcote is a browser built for fingerprint coherence
It is a Chromium fork, maintained by the same people who wrote this reference. It ships as a compiled browser rather than as a stealth script injected into someone else's — which is a description of how it is built, and is not an argument about how it behaves on this check.
This audit takes no position on how Clearcote scores on Capability surfaces checks, on this one, or anywhere else. It has no baseline corpus of other people's fingerprints to rank you against and no vendor scoreboard — nearly every check is self-referential, asking one browser the same question through two independent APIs and reporting whether both answers can be true at once. It runs identically on any browser, including ours. Run it on yours and read the result yourself.
See the other checks in Capability surfaces — the family css-surface-vs-realm belongs to.