I thought it was going to use ETAG based fingerprinting/tracking, which I always thought was pretty clever. The etag header is supposed to be used to control caching, so it's typically a server-side generated hash of the requested resource's content. But, there's no requirement for it to be, so you can generate a unique one, and the client will send it back to you next time it asks for that uri. Sort of like a cookie.
The fingerprint sources are here: https://github.com/fingerprintjs/blog-nojs-fingerprint-demo/... Basically some css that uniquely identifies some browsers, then headers like user-agent, language, etc.