Allow sites to opt into iframes having responsive sizing (sizing the <iframe> element in the parent document to the iframe document's layout overflow sizing, so that scrolling in the child document is avoided).
This is a natural feature to have for iframes, when the site wants to render the iframe content so that it looks seamless with the parent frame and avoids scrollbars.
Explainers: https://github.com/w3c/csswg-drafts/blob/main/css-sizing-4/responsive-iframes-explainer.md