[chc]

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.

[saagarjha]

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