单击时打开菜单

<div class="menu">
  <button class="menu-button">菜单</button>
  <ul class="menu-items">
    <li class="menu-item">选项1</li>
    <li class="menu-item">选项2</li>
    <li class="menu-item">选项3</li>
  </ul>
</div>

.menu {
  position: relative;
}

.menu-items {
  position: absolute;
  display: none;
}

.menu-button:focus + .menu-items,
.menu-items:focus {
  display: block;
}

const menuButton = document.querySelector('.menu-button');
const menuItems = document.querySelector('.menu-items');

menuButton.addEventListener('click', () => {
  menuItems.style.display = menuItems.style.display === 'block' ? 'none' : 'block';
});