What does my browser tell websites?

Every browser signal, with the call behind it

5 signals, none of which ask your permission first. The entropy column is this site's own estimate of how much each signal narrows a crowd, not a measured population statistic; the methodology page explains why that distinction matters.

Why almost everything here is a claim rather than a measurement

The browser signals are unusual among the six categories: they are the ones the browser volunteers about itself. Your screen size is a fact about hardware. Your IP address is a fact about routing. Your user agent is a sentence your browser wrote about itself, and it can write a different one whenever it likes.

That matters more than it sounds, because a great deal of the web is built on trusting it. Download pages pick an installer from it. Support articles branch on it. Analytics dashboards count it. All of that works because most people never change it, not because it is verifiable.

The one thing here that is genuinely measured rather than claimed is cookie support, and only because this page tests it properly: it writes a cookie, reads it back and deletes it, instead of asking the browser whether it thinks cookies work. Those two answers disagree more often than you would expect.

How the browser signals combine

On their own these are the weakest signals on the site. Hundreds of millions of people share a browser name. Most people on a given browser share a major version, because updates roll out to nearly everyone within a couple of weeks. An operating system family is one of about five answers.

The value to a tracker is in the joins and in the contradictions. A language list of three specific languages in a specific order is a much smaller crowd than any single language. A user agent claiming macOS on a device that draws text the way Windows does is a mismatch, and mismatches are more distinctive than either value would have been.

This is also why spoofing a browser identity so often backfires. Changing your user agent to something rare replaces a value you shared with millions by one you may share with nobody, and consistency checks against the other signals will notice.

What browsers have taken away here

This is the category where vendors have clawed the most back. Chrome's User-Agent Reduction freezes the minor version digits at zero and flattens the platform detail, moving the precise answer behind Client Hints that a site has to ask for and the browser can decline. Firefox rounds the version. Safari has reported a near-static string for years.

The direction of travel is consistent: turn passive leaks into explicit requests. A site that genuinely needs the full build number can ask for it, and the ask is visible and refusable. A site that was only collecting it because it was free gets nothing.