<button class="button">Button</button>

.button {
  padding: 15px 35px;
  background: none;
  border: 3px solid black;
  animation: 4s ease-in-out infinite borderColor;
  cursor: pointer;
}

@keyframes borderColor {
  0%, 55%, 100% { border-color: black }
  70%, 85% { border-color: yellow }
}