XHTML + CSS For Web Apps
The FeedLounge application UI is uses XHTML and CSS exclusively for the front end. While the XHTML does not currently validate due to our use of custom attributes (unless we write a custom DTD - which we probably will in the future), it was encouraging for me to see that you can build a complex web UI using XHTML + CSS.
This XHTML + CSS thing just might catch on, I tell ya. 
From what I can tell looking at the screen shots, it looks very clean and promising. Of course, it would be nice if colors and such are customizable.