Cases 1 & 2 are manageable through url parameters since there is shared infrastructure on both ends. These can be done without any user visible workflow changes

Cases 1 & 2 are manageable through url parameters since there is shared infrastructure on both ends. These can be done without any user visible workflow changes. Case 3 feels the most like a legitimate use case. Even with url parameters, there's no way for Disqus (or whatever) to know that a.com's user is the same person as b.com's user since those are independent entities. It is still solvable, but not in a zero-click way, by having a round-trip redirect (like an OAuth flow), to associate my user on the blog with my user on the commenting platform. But that does require that the blog track my session, which is not a requirement in the current world. On the other hand, I'm not sure how much I like having #3 work in general -- that a blog commenting platform can implicitly know what blogs I'm visiting, and potentially even having a blog on which I have no account implicitly having access to my account on other blogs, feels a bit intrusive to me. I'd rather have that integration at the level of a browser extension that modifies user agent behavior rather than something that happens without my permission.