回到顶部 css

<button id="back-to-top" class="back-to-top">回到顶部</button>

.back-to-top {
  display: none;
  position: fixed;
  bottom: 20px;
  right: 30px;
  z-index: 99;
  font-size: 18px;
  border: none;
  outline: none;
  background-color: #555;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

.back-to-top:hover {
  background-color: #777;
}

window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    document.getElementById("back-to-top").style.display = "block";
  } else {
    document.getElementById("back-to-top").style.display = "none";
  }
}

document.getElementById("back-to-top").addEventListener("click", function() {
  document.body.scrollTop = 0; // For Safari
  document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
});