3.3 Style components with CSS

BTW: this "className" attribute might change to just "class" in a future version of React, just as mentioned in a post by Dan Abramov, a member of the React team at Facebook:

you must use className instead of class to define the DOM css classes
You are completely right it’s annoying. It’s one of those early design decisions to align better with the DOM APIs that has proved to be confusing. We might change this in the future.