I think it's an imitation. They're not stock UI elements. For example, on my Mojave system, if I just make an HTML button, I can see it doesn't respect the systemwide dark setting, the highlight color looks slightly darker than a normal light-theme button and it doesn't invert the text color when highlighted.

Safari on windows were showing osx-styled default buttons, so I think safari also implement their own UI controls, not using system's ui elements.

Right. Browsers can't use the OS control implementations, because the demands of DOM/CSS simply can't be handled by those native implementations. But Firefox is trying to emulate the look and feel of the OS controls, using native theme drawing APIs when possible. It is very hard to get 100% right, which is why no browser does.