向下滚动时HTML / CSS中的静态菜单？

<nav>
  <ul>
    <li><a href="#section1">Section 1</a></li>
    <li><a href="#section2">Section 2</a></li>
    <li><a href="#section3">Section 3</a></li>
  </ul>
</nav>

nav {
  position: fixed;
  top: 0;
  width: 100%;
  background-color: #f1f1f1;
}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

li {
  display: inline-block;
  margin-right: 20px;
}

a {
  text-decoration: none;
  color: #333;
}

window.addEventListener('scroll', function() {
  var nav = document.querySelector('nav');
  if (window.scrollY > 100) {
    nav.classList.add('scrolled');
  } else {
    nav.classList.remove('scrolled');
  }
});