Ahmad Alfy shows how frame-sizing: content-height sizes an <iframe> to its content without postMessage scripts. The embedded page opts in with a responsive-embedded-sizing <meta> element present at load and calls window.requestResize() after content changes. Chromium-only for now, so keep @supports and a postMessage fallback.
alfy.blog/2026/10/09/iframe-that-finally-fit-their-content.html