Web technology is continuously getting better and more and more innovative websites are being created. HTML 5 replaced HTML 4 and it's supposed to bring more flexibility and to enhance a wide range of features. There are a lot of benefits this revamped language is offering to all web designers.
1. Offline Support There are certain features that come along with HTML5 that allow you to store more offline information which is a huge plus for mobile application developers. This will allow useful applications to continue to function even when there is no available internet connection.
2. Canvas and Video these features make it easier than ever before to add images and video to a web page and make it cross browser and mobile compatible.
3. GeoLocation API it isn't really part of HTML5 but since HTML5 is associated mainly with mobile device use, GeoLocation goes along with this discussion quite nicely. Developers can now more easily integrate user-specific location based information into an app and do things like that 'cool' augmented reality stuff people have been talking about. That's how it's possible to point your iPhone or Android device at a street and have it tell you where the closest coffee shop is.
4. Advanced Forms now your mobile browser can handle some of the background stuff that is required to make sure information that you enter in an online form is accurate instead of having to run additional scripts to do it. This really will speed up load times and increase that user experience stuff.
That being said, HTML5 sounds great and all developers should stop what they are doing right now and start using it shouldn't they? That's not necessarily the case.
5. The biggest issue is the acceptance (or lack thereof) of micro formats across multiple browsers. HTML5 uses a slew of new tags that add new 'semantic richness' to a web document but does not fully utilize micro formats, which makes HTML5's backwards compatibility non-existent. Being a graphic designer on the web, this is a bad thing.
Most of the other negative points start to get sort of complicated and difficult to explain so for now, I'll leave it at that. It is important to know that HTML5 is still a good thing -- it's just going to be a bit before it is completely integrated into every developer's day to day routine. As mobile internet browsing continues to grow, however, it will very soon have its place.