As far as amount of css goes, here's what you're dealing with to achieve something similar in only CSS.

  1. Button reset - to get browsers to the same starting point
  2. Rounded corners, Text shadows, and Gradients + vendoring
  3. Background image fallback
  4. Padding, Text size, line-height, etc
  5. Default, Hover, and Active states for all styles

That's a lot of css to write, effectively reduced to a single mixin. Of course, it's also great that you can use the bare minimum html.