Being able to pop this sort of creation inside an iframe on cohost without hosting it separately would make it tremendously more convenient to share projects, especially one-off goofy things. You can fit a lot of interactivity in well under the 5mb limit that currently applies for images, and iframes rule out most of the nasty shenanigans that are possible when running untrusted code.
I'd extremely be all about being able to embed a decker deck directly into a chost! Or like a twine game or similar. I think you'd be able to do lots of cool stuff with this!
I would absolutely love this feature, being able to check out html games without needing to go to a separate page from where I find out about them would be a magical experience.
I...would really rather this never happen. Security is the biggest concern, but even with CSS crimes we have the ability to slow down folks feeds to a crawl and in some cases break their rendering even just by mistake. This...there's just no way to implement this safely and to remove any chance of disrupting things of people just scrolling through their feeds (short of including a toggle to allow users to disable seeing them, and then that kind of removes the point).
As convenient as this is to have in-line, it's really best left to just being linked to on other sites.