Right now, an `<iframe>` element's content will always have an intrinsic sizing (otherwise known as the viewport of the iframe) that matches the `<iframe>`: if the `<iframe>`'s layout size changes, then the viewport bounds of the iframe document are adjusted accordingly, and the child-document receives a new layout. Two upcoming features will change this situation: 1. Auto-resizing iframes #1771. There are modes in this proposal where the iframe could specify a fixed intrinsic sizing. 2. [Fenced frames](https://github.com/shivanigithub/fenced-frame). These are iframes that are privacy-preserving, so once they load their contents cannot change size (or else there would be a communication channel to the parent frame via resize patterns). I propose that for such situations, we apply the usual replaced element semantics, with `object-fit` specifying how the contents are scaled and positioned within the rendering of the iframe. By default, `object-fit: contain` should apply, which will letterbox the iframe content. A polyfill demo demonstrating the rendering behavior is [here](https://output.jsbin.com/xixucun). The demo simulates a fencedframe, which when loaded freezes its size (try resizing with the resizer at the bottom-right).