Personally, I like the concept, but not the execution. It only changes its size externally on DOMContentLoaded, and again on load. So, if the user e.g. expands a <details> element, that won't update the size of the iframe. If the width of the main window changes size, resulting in the iframe changing width, resulting in the content within becoming a different height, it won't update the size of the iframe.

In cases like this, the iframe has to manually call window.requestResize(). And… that just doesn't feel like a big step up from the hacks we do today.

I remember <iframe seamless> being proposed many many years ago. I wonder why that was rejected.