I'm pretty sure that when you render just a <button> in at least Safari, the browser will render a native Cocoa button control. If you set a CSS property like background colour or change the border, then it will "fall back" to a custom rendered control that isn't from the OS UI.

I did a small bit of research into this, and found plenty of "anecdotal" evidence, but nothing confirming for sure. I'm looking and interacting with the controls and they seem pretty native - if they're a recreation than that's pretty impressive :)