html center button

  .center {
  display: flex;
  justify-content: center;
  
  align-items: center;
  height: 200px;
  border: 3px solid 
  green; 
} 

button{
    /*Change the width as much as you like, but make sure 
    margin-left and margin-right + width = 100%*/
    width:50%;
    margin-left:25%;
    margin-right:25%;
}