Browsers use the public suffix list to determine cookie scope. So .co.uk domains are just as isolated from each other as .com domains. You can even get your ow

Browsers use the public suffix list to determine cookie scope. So .co.uk domains are just as isolated from each other as .com domains. You can even get your own domains added to it, typically because you allow users to host their own content on a subdomain (like github.io for github pages). https://publicsuffix.org/ https://publicsuffix.org/