I have been dabbling with the new features that are available in HTML5. It brings HTML and Javascript to another level. Features such as Dragging and Dropping Files, Video, and Audio elements are very exciting as they eliminate the need for plug-ins. The one feature that I have been developing with is Canvas. I have wrote a few demos and games in 2D Canvas and WebGL (Some portions of this website render in Canvas depending on browser), which I am soon to launch as a series of tutorials. They are mostly in Javascript; even though it is best recommended to use the latest web browser, as Javascript has evolved in leaps and bounds over the last year as performance has improved so much. Javascript as a whole is not perfect yet, as it can slow down on some computers, or different HTML5 features are not fully supported in some browsers; yet you can see some major possibilities which will carry Web Development throughout this new exciting decade.
Javascript's speed for example is not it's biggest strength, but as I mentioned earlier, the new browsers will take on that issue and create a platform that can rival Adobe Flash in the long run. The thing I have enjoyed working with and keen to work more on is WebGL which is in technology devised by the Khronos Group; which is a 3D API standard similar to Khronos' Desktop 3D API OpenGL. With the possibility this brings to develop hardware rendered graphics via the web is going to be stunning for those 3D graphics connoisseur and enthusiasts (I'm waiting for more web browsers to support these features fully!).
I am currently writing tools and tutorials to accompany this site on both 2D Canvas Game development and WebGL Graphics programming at this current time (February 2011). I am testing the web apps on Google Chrome Beta, as Firefox 4, has yet to become a stable release at this time of writing thus does not support WebGL as Chrome does. Once Firefox 4 is ready, then I know the best time to launch these tutorials will be then.
Other portions of HTML5 will be covered in later tutorials which will be available over the next couple of months during Summer 2011. Stay Tuned and you will be filled in.
Examples linked above courtesy of HTML5 Demos.