It was fairly easy to communicate between frames with JavaScript, so before this was widely adopted, I remember the way we used to “fetch” new data was with an

It was fairly easy to communicate between frames with JavaScript, so before this was widely adopted, I remember the way we used to “fetch” new data was with an iframe that always had a meta refresh tag in it, and the server response would include JavaScript that would call into functions from the main window passing along any data that was needed. I might be wrong, but I think even early versions of Gmail did something similar.