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It's true that I could be lacking imagination. Because to me the portal proposal reads like it's simply moving the implementation of Google's AMP carousel into the browser in order to solve the address-bar problem. Your suggested use cases aren't terribly convincing, I'm afraid. Zooming UIs are perfectly possible using existing web technology. It would be interesting to have a standard way to embed Twitter/Facebook-s…
Sorry about that, I didn't mean you have little imagination, but rather that any reading of a tech spec requires a bit of imagination. It doesn't seem all that unreasonable for a web page to show a preview if the window size is small enough. You could do that with a media query. And then when it goes full-screen, it would already be loaded and that would just be a resize. This would take cooperation from web sites.
Building richer cross-site interactions into web standards and browser interfaces is an intriguing idea. A more dynamic model of hypertext, where the browser intelligently displays linked-to content. Even though portals are a highly limited version of that idea, Google has a high chance of getting them standardized and implemented by competing engines, so hopefully more useful applications can be found.