To understand this better, here's a good place to start in the Webkit source code: https://github.com/WebKit/webkit/blob/master/Source/WebCore/... That is the interface Webkit uses to draw "native theme" buttons and other controls on Mac. It is not using a real Cocoa NSButton for each button in the Web page. Instead, it creates its own internal DOM and CSS rendering objects (HTMLButtonElement, RenderButton respectively) which are cross-platform code. Every time it needs to draw a button, it calls virtual methods on RenderTheme. On Mac, the RenderThemeMac implementation calls down into Cocoa code to draw the background and borders of the button. (But not text; that wouldn't work in general, because in HTML a <button> can contain arbitrary HTML elements, and native buttons can't handle that.) That's pretty much the only involvement of Cocoa native button code. Firefox works in a very similar way; here's the Firefox version of RenderThemeMac: https://github.com/mozilla/gecko-dev/blob/master/widget/coco...

You can't tell by looking at the controls whether Safari is using the native control implementation (it isn't). All you can tell is that the Webkit implementation looks like and acts like the native implementation.