Browsers are becoming an increasingly capable platform for application development. One example is that WebGL-based 3D graphics are supported by many of the latest browser versions. Palm has taken this idea to its extreme: On Palm devices, all applications are web applications. Recently, Palm has given a demo of Enyo, the next generation of its development framework. A blog post gives a few details on it. One cool feature (at 15:53 in the video linked to from the post) is that Enyo user interfaces scale seamlessly from small screens (cell phones) to large screens (tablets, desktops). In Ares, Enyo has a web-based IDE. Enyo also runs in normal web browsers which allows one to develop in a browser and then push the application to a device. In the video, the presenter mentions that they only push for performance tests. Enyo is a very early demo and only gives a preliminary glimpse of the final product. From the video it sounds as if it is currently WebKit-only. It would be cool if that could change in the future.
Update 2011-01-24: The ultimate CSS layout spec for webapps