4 Simple Transitions of Button

4 Simple CSS Transition to Enchance Your Buttons

Darken

Set a darker shade background color when :hover is activated

Fade Out

Set opacity to 1 by default, and reduce the opacity when :hover is activated.

Change Color

Inset Border

To create an inset of border,

button: hover{
  box-shadow: inset 0 0 0 5px #darkderColor;
}