Transition questions

please see

first, why are there 2 background-color ?

second, why does this code work without a transition property value? what is a transition property?

Because there are two background-colors defined: Pink for the initial background-color of the button and limegreen for the hovered state of the button.

It does have a transition-property: background-color. The transition property defines on what css property the transition is applied. With this definition, the transition of size or font color would not be smooth.