A commenter above mentioned that when you target an `iframe` it sends back the header `Sec-Fetch-Dest: iframe`, so if it doesn't have that then you know it requ

A commenter above mentioned that when you target an `iframe` it sends back the header `Sec-Fetch-Dest: iframe`, so if it doesn't have that then you know it requires the full page and not the snippet. So, yes, easy no JS fall back by dynamically adding the `base` tag. Nothing else needed.