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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.

This is a good point. It also makes reasoning about classlists more clean, if all are guaranteed to be managed by your own code.

That caught my attention immediately. It just seems like a hacky, roundabout way to avoid using classes for different button styles, which to be clear, are an example of exactly why classes exist.

It's mainly the latter. I did consider just using a class, but ultimately settled on this solution. This approach might facilitate usage with additional CSS compilers/tooling.

I admit I become hesitant when I see "data-*" attributes as it makes me think Javascript might be necessary to use it

"data-*" attributes don't require any JavaScript; their values can even be accessed from CSS with `content: attr(data-*);`