> How can you prevent it from accessing a parent doc when its not a separate document.

By turning it into a document boundary when you use the sandbox attribute, kinda similar to loading an svg file inside of an <img> tag.

and yeah you could get 90% of the way there with an iframe srcdoc, but I was imagining some kind of cross between an <iframe> sandboxed into its own origin, and an <img> where it still has its own intrinsic size.

but it was mainly just a throw away thought, I've not really thought it through much deeper than that.