It makes perfect sense that HTML5/JS would overtake traditional UI frameworks on the client side. Doing Win32, Java Swing, etc. was a horrible experience compared to the relative niceness of HTML/JS.

Adobe Flex/AIR was probably the best client UI dev experience I've had. MXML and Actionscript 3 is VERY similar to HTML/JS. Given the benefits of so many platforms moving towards HTML/JS as a kind of common language for UI development, these are exciting times.