.container{
  position: relative;
  padding: 150px;
  background-color: #000;
}
.envelope{
  width: 80px;
  height: 50px;
  border: 3px solid #fff;
	position: relative;
  overflow: hidden;
  &:before, &:after{
    content:" ";
    position: absolute;
    left: 2px;
    width: 70px;
    height: 70px;
    border:3px solid #fff;
    transform:rotate(45deg);
    top:-42px;
    -webkit-border-radius: 5px;
    -moz-border-radius: 5px;
    border-radius: 10px;
  }

    animation: flip 2.5s infinite;

}

@keyframes flip {
  0% {
   transform: perspective(100px) rotate3d(0, 1, 0, -360deg);
    animation-timing-function: linear;
  }

  25% {
   transform: perspective(100px) rotate3d(0, 1, 0, -260deg);
    animation-timing-function: linear;
  }

  50% {
   transform: perspective(100px) rotate3d(0, 1, 0, -170deg);
    animation-timing-function: linear;
  }
  
  75% {
   transform: perspective(100px) rotate3d(0, 1, 0, -80deg);
    animation-timing-function: linear;
  }

  100% {
    transform: perspective(100px) rotate3d(0, 1, 0, 0);
    animation-timing-function: linear;
  }
}