<div id="testbutton"></div>
body {
  height:100vh;
  width:100vw;
  background-color:rgb(19, 20, 22);
  overflow:hidden;
  font-family: Lato, "Trebuchet MS", "Verdana", "sans-serif";
}

#testbutton {
  width: 150px;
  height:50px;
  border-radius:180px;
  position:relative;
  left:calc(50% - 75px);
  top:calc(50% - 25px);
  background: linear-gradient(60deg, #f79533, #f37055, #ef4e7b, #a166ab, #5073b8, #1098ad, #07b39b, #6fba82);
  cursor:pointer;
  line-height:12px;
}

#testbutton:before {
  content:'';
  z-index:1;
  position:absolute;
  display:block;
  width:80%;
  height:70%;
  top:15%;
  left:10%;
  transition: 0.3s opacity ease-in-out;
  filter:blur(15px);
  opacity:0;
  background: linear-gradient(60deg, #f79533, #f37055, #ef4e7b, #a166ab, #5073b8, #1098ad, #07b39b, #6fba82);
}

#testbutton:hover:before {
  opacity:1;
  transition: 0.3s opacity ease-in-out;
  filter:blur(25px);
  background: linear-gradient(60deg, #f79533, #f37055, #ef4e7b, #a166ab, #5073b8, #1098ad, #07b39b, #6fba82);  
}

#testbutton:after {
  content:'CLICK ME!';
  text-align:center;
  line-height:40px;
  font-size:18px;
  color:rgba(235,235,235,1);
  font-weight:bold;
  z-index:5;
  position:absolute;
  display:block;
  border-radius:180px;
  width:92%;
  height:80%;
  top:10%;
  left:4%;  
  background-color:rgb(19, 20, 22);
}

External CSS

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External JavaScript

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