Internet Explorer 5 introduced XMLHttpRequest (ActiveXObject) in 2001.
I might be wrong, but I think even early versions of Gmail did something similar.
Started in 2000 in Exchange/Outlook web and took a year or two to become prevalent in the browsers and other places.
https://medium.com/@mohamedtayee3/make-an-http-request-in-ja...
Prototype.js and jQuery had a big hand in the latter.
The big inbetween OWA and Gmail, was Oddpost, which was also IE only, and was purchased by Yahoo three months after gmails April Fool announcement, to become Yahoo Mail. In a roundabout way, you can argue gmail was a copy of what was eventually Yahoo Mail.