<div id="button" class="css-button">
  support CSS
</div>
<div id="button" class="gsap-button">
  support GSAP
</div>

@import url(https://fonts.googleapis.com/css?family=Open+Sans);
html, body {
  font-family: 'Open Sans';
  width: 100%;
  height: 100vh;
  margin: 0;
  padding: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: #1E1F26;
}
#button {
  margin: 4%;
  padding: 1% 4%;
  border-radius: 1px;
  background: blck;
  color: white;
  font-size: 16px;
  border: 1px solid white;
  cursor: pointer;
}
.css-button {
  animation: inout 1s ease infinite;
}

.gsap-button {
  border: 1px solid #82c303 !important;
}

@keyframes inout {
  0% {
    -webkit-box-shadow: 0 0 0 0px rgba(255,255,255,0.4);
    -moz-box-shadow: 0 0 0 0px rgba(255,255,255,0.4);
    box-shadow: 0 0 0 0px rgba(255,255,255,0.4);
  }
  100% {
    -webkit-box-shadow: 0 0 0 20px rgba(255,255,255,0);
    -moz-box-shadow: 0 0 0 20px rgba(255,255,255,0);
    box-shadow: 0 0 0 20px rgba(255,255,255,0);
  }
}

gsap.fromTo(".gsap-button", {
  boxShadow: "0 0 0 0px rgba(130, 195, 3, 0.4)"
}, {
  boxShadow: "0 0 0 20px rgba(130, 195, 3, 0)",
  repeat: -1,
  duration: 1
});