<a href="/" class="bn13">Button</a>

.bn13 {
  display: inline-block;
  padding: 0.65em 1.6em;
  border: 3px solid #ffffff;
  margin: 0 0.3em 0.3em 0;
  border-radius: 0.12em;
  box-sizing: border-box;
  text-decoration: none;
  font-family: "Roboto", sans-serif;
  font-weight: 300;
  color: #ffffff;
  text-align: center;
  transition: all 0.2s;
  animation: bn13bouncy 5s infinite linear;
  position: relative;
}

.bn13:hover {
  background-color: white;
  color: #000000;
}

@keyframes bn13bouncy {
  0% {
    top: 0em;
  }
  40% {
    top: 0em;
  }
  43% {
    top: -0.9em;
  }
  46% {
    top: 0em;
  }
  48% {
    top: -0.4em;
  }
  50% {
    top: 0em;
  }
  100% {
    top: 0em;
  }
}