Firstly, let me say that this looks great. I really like the classless approach. One minor question/nitpick: > To make a button primary, add the data-primary attribute to it. For things like this, I feel like a class would be ok while still calling the library "classless". Is there some advantage to using a data-* attribute here or is it just to retain "purity" of the classless tagline?
I like the distinction. It separates effects applied by the library from affects applied by your custom class, since you can still add a class if you like. At a glance, I know where the effects are coming from.