DHH is (obviously) David Heinemeier Hansson (the creator of Rails) and RIA (in this context) means Rich Internet Applications. And here is the quote:

I think there’s always room for new ideas, but I don’t think that the whole fuss that’s currently going on about RIA, rich Internet applications, is justified. I think we’ve been through this cycle so many times before that it in some ways amazes me how history seems to be ignored. We went through this with Java applets, they were going to rule the Web. Everything was going to be in a Java applet. HTML and CSS is history. And Flash came around, and Flash started focusing on applications. Now Flash is going to rule the Web and HTML and so on is yesterday. Now, Silverlight, Apollo, JavaFX, they’re all bidding to take over the JavaScript, HTML and CSS [spaces], and I just don’t buy it. I don’t buy that developers by and large are going to jump into a proprietary technology and replace what HTML and CSS has given them.

Read the full interview here