<div></div>

div {
  width: 0px;
  height: 0px;
  background: lime;
  border: 10px solid transparent;
  border-bottom-color: transparent;
  border-right-color: transparent;
  border-left-color: transparent;
  background: transparent;
  animation: myAnimation 1s infinite;
}


@keyframes myAnimation {
  25% { 
    border-top-color:  transparent;
    border-right-color: green;
    border-bottom-color: transparent;
    border-left-color: transparent;
  }
  50% { 
    border-top-color:  transparent;
    border-right-color: transparent;
    border-bottom-color: green;
    border-left-color: transparent;
  }
  75% {
    border-top-color:  transparent;
    border-right-color: transparent;
    border-bottom-color: transparent;
    border-left-color: green;
  }
  100%   { 
    border-top-color:  green;
    border-right-color: transparent;
    border-bottom-color: transparent;
    border-left-color: transparent;
  }
}