The reason for a new element, rather than just tacking this behavior onto `iframe` seems to be backwards compatibility [1]. > We wish to give user agents as mu

The reason for a new element, rather than just tacking this behavior onto `iframe` seems to be backwards compatibility [1]. > We wish to give user agents as much flexibility as possible to isolate the host and guest browsing contexts (in implementations, in separate processes), even when the active documents may be same origin-domain. To make this possible, we intend not to expose the Document or WindowProxy of the guest contents, via the IDL attributes on HTMLIFrameElement, by using access to named windows, or by indexed access to window.frames. Without such access, communication can be be guaranteed to be asynchronous and not require shared access to JavaScript objects. Those operations which don't apply to portal contexts would all need to be modified to throw an appropriate exception in such cases. [1]: https://github.com/WICG/portals/blob/master/explainer.md https://github.com/WICG/portals/blob/master/explainer.md