As a wider range of devices are able to access the internet, designing a website to work everywhere becomes more and more challenging. Desktop design is pretty much covered - it's the standard. Small screen design is slowly increasing in uptake, but it's still usually a second-thought, rather than a prime consideration. Unless a website is intended primarily for small-screen use. Now we're getting devices in between, such as tablets. Add with most of these new devices comes touch.
PPk's investigations and comments on touch support in mobile browsers shows a few things - more work is needed to make touch really work with websites, and web site developer will need to start supporting more than just mouse events. There are now three event types that need to be taken into account: mouse, keyboard and touch.
This area may be much more important than some other parts of HTML5. It is interesting to note that although Firefox and Opera are moving quite quickly in some area regarding HTML5 and added features, they score badly on touch and mobile browser UI support.