http://dds6qkxpeiqvogt4y2fd7vtsxq6pvokekb2h4la7jd4iwl4ljuawu5qd.onion/wiki/Browser_Tests
These sites generally check for various browser characteristics as well as enabled functionality to reveal specific identifiers. Many of these identifiers require JavaScript, but reports commonly include elements like HTTP headers and attributes, the User Agent, numerous JavaScript attributes, IP address, operating system indicators, browser plugins, cookies, cache, browser window dimensions, fonts and so on.