Riot.js is a micro framework. It's only three and a half kilobytes. With it you get custom tags, eventing, and rounding, that's all. Otherwise it just gets out of your way and lets you code. I was able to learn the entirety of Riot.js in about two days, one day to read the docs, and one day to write a small app. The ability to get started quickly, and for such a small size is why I think Riot could become a very popular framework.
This is implied, so I don't have to write this.what, I can just write what. I'll add my file to my index file as a script of type = Riot/tag. You also have to explicitly mount any custom tag used outside of a .tag file. For example, here in the body of my index. You just say riot.mount and then you can pass a * if you want to mount every custom tag on the page, or you can pass in specific tags.
Now I'm free to use my Hello World component in my index file. When I refresh, you'll see that we've successfully created a simple Riot component, complete with databinding and runtime compilation.