:root {
  scroll-behavior: smooth;
}

body {
  width: 50%;
  max-width: 800px;
  margin: 0 auto;
}

p {
  font-family: sans-serif;
  font-size: 1.3rem;
  line-height: 1.5;
}

div {
  position: fixed;
  top: 0px;
  left: 0px;
  right: 0px;
  background: gray;
  padding: 10px;
}

.fade-out {
  animation: fade-out 0.3s linear both;
}

.fade-in {
  animation: fade-in 0.3s linear both;
}

@keyframes fade-out {
  from {
    opacity: 1;
  }

  to {
    opacity: 0;
  }
}

@keyframes fade-in {
  from {
    opacity: 0;
  }

  to {
    opacity: 1;
  }
}