> Furthermore, portals can also overwrite the main URL address bar, meaning they are useful as a navigation system
Isn't this already possible with iframes and target="_top" / target="_parent"? You can also use the <base>-Tag inside the iframe.
> The advantage over using Portals over classic links is that the content inside portals can be pre-loaded while the user scrolls through a page, and be ready to expand into a new page without having the user wait for it to load.
What? What about the vast majority of links that appear in-line, in text (think Wikipedia). What about the increased bandwidth usage for pre-loaded pages I don't intend to visit? What about the obvious tracking issues? What about the further increased difficulty for users to even tell which page they are actually on, if the transition to another page is now smooth and not clear-cut?
This article makes it seem like <portal> magically removes all the security concerns people had for decades with iframes, which imho is not the case at all.