body {
  margin: 0;
  background-color: #000;

  font-family: Helvetica, Sans-serif;
}
main {
  min-height: 100vh;
  display: flex;
  align-items: center;
  flex-flow: column;
  justify-content: center;
}
div {
  display: flex;
  margin: 2.5vw;
  padding: 0.2vw 5vw;
  background: #000;
  border-radius: 5px;
}
h1 {
  padding: 0.3vw 1vw;
  font-size: 3vw;
  color: #fff;
}
button {
  text-decoration: none;
  color: #fff;
  border: none;
  outline: 0;
}
.rainbow-button {
  width: calc(36vw + 6px);
  height: calc(8vw + 6px);
  background-image: linear-gradient(
    90deg,
    #00c0ff 0%,
    #ffcf00 49%,
    #fc4f4f 80%,
    #00c0ff 100%
  );
  border-radius: 5px;
  display: flex;
  align-items: center;
  justify-content: center;
  text-transform: uppercase;
  font-size: 2vw;
  font-weight: bold;
  position: relative;
}
.rainbow-button:after {
  content: 'click to change color';
  width: 35.5vw;
  height: 7.5vw;
  background-color: #000;
  border-radius: 5px;
  display: flex;
  align-items: center;
  justify-content: center;
  position: absolute;
}
.rainbow-button:hover {
  animation: slidebg 2s linear infinite;
}
@keyframes slidebg {
  to {
    background-position: 40vw;
  }
}