[jeroenhd]

There's absolutely nothing stopping browser vendors from implementing ActivityPub. I don't think the browser is the right place, but browsers also implemented automatic RSS feed detection for more than a decade so it's not exactly unprecedented either.

Browsers don't need to care, anyway. There are some problems that need solving (notably, following/interacting with users on other platforms) but there are ways to do that. I'm not quite sure why nobody has come up with a good standard for a web-activitypub: URL handler, but once everyone agrees on a format adding support for cross-server stuff should be easy.

Web browsers can access webmail clients and mailing lists just fine. They even have mailto: as a means to cross the federation gap. All of these problems are implementation issues, not conceptual impossibilities.