Found some context: https://github.com/WICG/portals

I think this proposal is a better starting point for readers than an API spec.

Goals, taken directly from the proposal:

- Enable seamless navigations from a page showing a portal, to the portaled page

- Enable seamless navigations between pages of a portal-aware website

- Avoid the characteristics of iframes which have negative impacts on security, privacy, and performance

Also: https://github.com/WICG/portals#summary-of-differences-betwe...