Prerenders cross-origin iframes with an opt-in response header. Browsers will now prerender all cross-origin frames if the top-level frame's HTTP response includes the Supports-Loading-Mode: prerender-cross-origin-frames.
By default, navigational prerendering delays the loading of all cross-origin iframes until the referring page activates the prerendered page. However, there are some cases where cross-origin iframes are particularly important to an application, such that delaying them negates many of the benefits of prerendering.
Explainers: https://github.com/WICG/nav-speculation/blob/main/prerendering-cross-origin-iframes.md