押し込まれたボタン　:active

<div id="btn">START</div>
#btn {
  display: inline-block;
  background-color: aqua;
  padding: 10px;
  border-radius: 4px;
  box-shadow: 0 5px 0 #5454e6;
  user-select: none;
}
#btn:active {
  margin-top: 5px;
  box-shadow: none;
}