It's like an iframe, but can be navigated into.
Explainer: https://github.com/WICG/portals/blob/master/explainer.md Editor's spec draft is here: https://wicg.github.io/portals/
As of today (Mar 24, 2019) you'll need to use Chrome Canary with --enable-features=Portals
command line flags.