Hah I was hoping someone would bring this up..

We were using that XMLHTTP object as far back as 1999 (I just went to look at old source code) to load page fragments do quasi-SPA stuff before it was a thing.

Same - I recall using it heavily in 99. But that was just on one internal app. I admit that I didn't really predict the SPA thing until Google maps came out and really showed how far it could be taken.

I worked for an ISP in the late 90s and we built a call center app using IE6, SSI, XMLHttp and also IE's built in CSS classes that matched the Windows colours to essentially have a UI that looked and felt like a Windows 95 app..

It was completely unnecessary (at least the look and feel part) but the ability to load in new information into the UI without a full refresh was great.

At the time this was running on Apache 1.1 via Perl CGIs and backed by an mSQL database (that was single-threaded!) so we wanted to minimize complex SQL queries as much as possible..

We eventually moved to a system that would batch pre-render customer details and call history to static HTML overnight when our call center was closed, to reduce the startup time of bringing up a customer's info when the agent had them on the phone. Then for that day we'd use XMLHttp to dynamically append new information from the DB (using a limit statement) and then re-build the static files the next night etc..

In hindsight, we did some creative problem solving for a bunch of self-taught 20-year-olds in the 90s.

That one is truly ancient, and unique in that it actually uses XML, and even SOAP, in AJAX calls. (Although the SOAP is mostly a façade and a fancy way to call a function without any arguments.)