You are right, and with a server-side component you could fetch arbitrary content, the endpoint could look something like example.com/transclude/a.b.com/section/page?path=2a14n which would make the server download a.b.com/section/page, extract the HTML element which is the 13th ('n') child of the 14th child of the first ('a') child of the second child of the root element, and return it.
I’m sure there are implementations of this already.
I’m sure there are implementations of this already.