[dimal]

This isn’t programming. It’s transclusion[0]. Essentially, iframes and images are already forms of transclusion, so why not transclude html and have the iframe expand to fit the content?

As I wrote that, I realized there could be cumulative layout shift, so that’s an argument against. To avoid that, the browser would have to download all transcluded content before rendering. In the past, this would have been a dealbreaker, but maybe it’s more feasible now with http multiplexing.

[0] https://en.m.wikipedia.org/wiki/Transclusion#Client-side_HTM...

[PoignardAzur]

With Early Hints (HTTP code 103), it seems especially feasible. You can start downloading the included content one round-trip after the first byte is sent.