The dojo toolkit is very useful for developing HTML5 apps. It's easy to use and it has some great features like dojox-mobile, which makes building mobile apps very simple as well as having a very powerful library of widgets for creating rich user experiences. It's also very fast, so if you're doing any heavy computation or rendering in your application, it's good to use this instead of vanilla JS. There are many different frameworks out there, but it's hard to choose one when you're just starting off with an HTML5 project. I've used dojo to create HTML5 apps such as games, interactive charts, and even a custom dashboard for my own business.