Re: [w3ctag/design-reviews] Portals (#331)

To @kenchris's point, I want to note that in my role as HTML spec editor, I've been really, really happy and excited about the idea of portals as "iframes done right". Although popular articles or explainers may focus on the flashy features like transitioning the URL bar or adopting your predecessor, @jeremyroman and others have put a lot of thought into the foundations. For a spec that is still in the experimentation stages, it's quite solid and integrates well with existing primitives.

For an example of what I mean by "iframes done right", iframes have many features that complicate their model. Those features can be convenient at times, but in my opinion don't pull their weight for the complexity they add.

The biggest of these is synchronous access to the framed document. This has caused huge spec complexity, security, and implementation hurdles, which we are [still sorting out today](https://blog.whatwg.org/windowproxy-window-and-location). Portals omit that, in favor of simple postMessage-based communication. Other such features are things like srcdoc, the observable initial about:blank transition, the weird load event ([still buggy](https://github.com/whatwg/html/issues/4292)), etc.

I do think there are still some things that need porting, such as the story around feature policy/sandbox. But even that will be a simplification opportunity, most likely; instead of allow + allowfullscreen + allowpaymentrequest + sandbox, I'm hopeful we'll have something more coherent and unified.

Finally, the biggest missing piece is session history integration, tracked at https://github.com/WICG/portals/issues/19. That will affect things like "fully active", indeed. This is complicated by [known issues with the session history spec](https://github.com/whatwg/html/issues/1454). Perhaps more generally, there are probably other things where the notion of "nested browsing contexts" should apply to portal browsing contexts as well (e.g. [update the rendering](https://html.spec.whatwg.org/#update-the-rendering)?). Portals add more complexity here on top of iframes due to the activation possibilities. Still, I am confident that because portals allow us to start with a clean slate, and because they decrease the synchronous observable interactions, the editors will be able to find and spec a model that works well.

I hope this helps!

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