To be clear, ATT is the system Apple put in place for native apps that is enforced by the App Store review process. When it comes to the browser, I think you’re more interested in Intelligent Tracking Prevention (ITP).

You should check out WebKit‘s tracking prevention policy:

https://webkit.org/tracking-prevention-policy/

Also the Privacy category on the WebKit blog:

https://webkit.org/blog/category/privacy/

Specific blog posts you might be interested in are:

https://webkit.org/blog/7675/intelligent-tracking-prevention...

https://webkit.org/blog/9521/intelligent-tracking-prevention...

https://webkit.org/blog/10218/full-third-party-cookie-blocki...

https://webkit.org/blog/11338/cname-cloaking-and-bounce-trac...

https://webkit.org/blog/11545/updates-to-the-storage-access-...

https://webkit.org/blog/15697/private-browsing-2-0/