Two important things happened to make the Ajax Revolution possible. The first was that Netscape failed to deliver Navigator 5. They did deliver Navigator 6 and 7, but they were so unstable that they failed to get much adoptions. So use of Netscape Navigator slowly declined as old computers that had it installed failed or were replaced. The second was that Microsoft disbanded the Internet Explorer team thinking that the Browser War was over. They put IE6 into maintenance, and put all of their creative energy into Dot Net, WPF, and the failure that would become Silverlight.
The consequence of this neglect of the web was that IE6, which was the best web browser the world had ever seen*, took an enormous share of the market. Now a web developer had only one version to support. The huge catalog of bugs and incompatibilities shrank down to just the final IE6 bugs, which was an infinitely smaller bugset than we faced in 2000. The web had become stable enough that we could now profitable write for it. Ajax was not a new idea. I was doing it in 2000. There were other arrowbacks who were doing it as early as 1996.
Things were better, but the DOM had not been fixed. I recommended avoiding the DOM, accessing it indirectly through popular libraries. The libraries could hide much of the horror that is the DOM. There were many of these libraries, including a few that I had written myself. They all provided a more pleasant and productive experience. The most difficult part about adopting an Ajax library was selecting the right one.
That is because the web standards movement tormented Microsoft and other browser makers to improve the quality of their browsers. WHATWG, frustrated with the inaction of W3C, started drafting its own standard. While I disagreed with much of it, there was no doubt that it was better thought out than what W3C had been publishing.
So I have changed my mind about the DOM. It is still awful, but it is much better than the popular alternatives.
*Yes, it is hard to imagine that IE6 was the best browser that the WWW had ever seen. That is just how bad the WWW was in 2000.