Netscape 4 has this with inflow layers — `<ILAYER SRC=included.html></ILAYER>`
https://web.archive.org/web/19970630074729fw_/http://develop...
https://web.archive.org/web/19970630094813fw_/http://develop...
I always wondered why it was called ILAYER. Ty
As far as I'm aware of it, changing the SRC-attribute was quite crash-y and the functionality was stripped soon. (I remember playing with this in beta, and then it was gone in the production version.)