The other is what could be described as "focus" or "modality". A lot of people use windows as a poor man's workspace. For example, if I'm researching where to travel, I'll have a window open with tons of tabs — Google Flights, Kayak, Tripadvisor, Booking.com, all jammed into one "workspace". If I'm comparison shopping for one specific thing, there'd be Amazon, Jet, Etsy, eBay, etc. This is how people tend to work: Windows separate modes, tabs separate units of focus within that mode. And yet there's no browser that supports such a way of working. We have to accept that people now "live" within the Internet, and need to support different modes. Rather than think of the browser as a shell, why not think of the browser as a world in which you can open up different types of interacting with that world? For example, a Google Doc tab is not something that works very well as a tab. It's a document, representing its own modality. Slack is an app, also its own modality. A "workspace" is something else again. And so on. These are all "applications" in a classical sense, yet work within the context of a user.
I'm not suggesting a ChromeOS, just that what we think of as "the web" could be rethought in terms of different variations — ways to work — with the same thing.
Also, in terms of privacy, why are we still using cookies? Why are we "logging in" and "logging out" of web sites and having password managers that automatically fill out form fields? Why can't the web site negotiate its session with the browser ("I want an identity for gmail.com, please give me one"), which can already know who I am?