纯CSS实现响应式导航栏的下拉菜单效果的实现步骤

<nav class="navbar">
  <ul class="nav-menu">
    <li class="nav-item">首页</li>
    <li class="nav-item with-dropdown">
      产品
      <ul class="dropdown">
        <li class="dropdown-item">产品1</li>
        <li class="dropdown-item">产品2</li>
        <li class="dropdown-item">产品3</li>
      </ul>
    </li>
    <li class="nav-item">关于我们</li>
    <li class="nav-item">联系我们</li>
  </ul>
</nav>

.navbar {
  background-color: #f8f8f8;
  padding: 10px;
}

.nav-menu {
  display: flex;
  justify-content: space-between;
  list-style: none;
  margin: 0;
  padding: 0;
}

.nav-item {
  padding: 10px;
}

.with-dropdown {
  position: relative;
}

.dropdown {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: white;
  padding: 10px;
}

.dropdown-item {
  padding: 10px;
}

.with-dropdown:hover .dropdown {
  display: block;
}

var dropdowns = document.querySelectorAll('.with-dropdown');
dropdowns.forEach(function (dropdown) {
  dropdown.addEventListener('click', function () {
    this.classList.toggle('active');
    var dropdownMenu = this.querySelector('.dropdown');
    dropdownMenu.classList.toggle('open');
  });
});

@media screen and (max-width: 768px) {
  .nav-menu {
    flex-direction: column;
    align-items: stretch;
  }

  .nav-item {
    display: none;
    padding: 10px;
  }

  .navbar.open .nav-item {
    display: block;
  }

  .navbar-button {
    display: block;
    background-color: #f8f8f8;
    border: none;
    padding: 10px;
    cursor: pointer;
  }
}

<nav class="navbar">
  <button class="navbar-button">菜单</button>

  <ul class="nav-menu">
    <li class="nav-item">首页</li>
    <li class="nav-item with-dropdown">
      产品
      <ul class="dropdown">
        <li class="dropdown-item">产品1</li>
        <li class="dropdown-item">产品2</li>
        <li class="dropdown-item">产品3</li>
      </ul>
    </li>
    <li class="nav-item">关于我们</li>
    <li class="nav-item">联系我们</li>
  </ul>
</nav>