style.css

#container {
  display: flex;
}

#box1 {
  background: pink;
  width: 100px;
  height: 100px;
  margin-right: 20px;
  text-align: center;
  line-height: 96px;
  user-select: none;
}

#box2 {
  background: pink;
  width: 100px;
  height: 100px;
  margin-right: 20px;
  text-align: center;
  line-height: 96px;
  user-select: none;
}

#box3 {
  background: pink;
  width: 100px;
  height: 100px;
  margin-right: 20px;
  text-align: center;
  line-height: 96px;
  user-select: none;
}

.box{
  animation: flash 1s linear ;
  width:50px;
  height:50px;
  background:pink;
}

@keyframes flash {
  0% {
    background:orange;
  }

  99% {
   background:orange;
  }


  100% {
    background:pink;
  }
}