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.

In dark mode, buttons have a darker highlight and don't invert their text color.

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.

That’s because Safari was ported to Windows by porting a large chunk of AppKit to Windows - it didn’t make direct use of Win32.

Wow this is interesting! Does that means technically we could have any OSX app compiled for windows while still retaining OSX-like appearance? Did Apple ever released this AppKit windows port for others to use? Does apple still maintaining the port for MacOS as well or did it die along with Safari for windows?

Before NeXT was acquired by Apple, they had OpenSTEP running on top of Windows. Back in the day, you could run a NeXTSTEP app on NeXT hardware, Sun hardware as well as x86: https://en.wikipedia.org/wiki/OpenStep#OPENSTEP_Enterprisee

It was originally ported for and is still used in iTunes.

iTunes uses CoreFoundation but it kinda barely uses AppKit even on macOS.

Safari buttons on macOS are native unless you style them in a way that causes their font or size to change.