如何html+css+js模拟select+option下拉菜单

HTML下拉菜单通常使用select和option元素组合实现，CSS可以用来样式化下拉菜单，JS可以用来实现下拉菜单的交互和动态更新。

以下是一个简单的HTML+CSS+JS下拉菜单实现示例：

HTML部分：

<div class="dropdown">
  <button class="dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    请选择
  </button>
  <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
    <a class="dropdown-item" href="#">选项1</a>
    <a class="dropdown-item" href="#">选项2</a>
    <a class="dropdown-item" href="#">选项3</a>
  </div>
</div>

CSS部分：

.dropdown-toggle {
  display: block;
  width: 100%;
  padding: .375rem .75rem;
  font-size: 1rem;
  font-weight: 400;
  line-height: 1.5;
  color: #212529;
  background-color: #fff;
  background-clip: padding-box;
  border: 1px solid #ced4da;
  border-radius: .25rem;
  transition: border-color .15s ease-in-out,box-shadow .15s ease-in-out;
}

.dropdown-toggle:focus {
  outline: 0;
  box-shadow: 0 0 0 .2rem rgba(0,123,255,.25);
  border-color: #80bdff;
}

.dropdown-menu {
  position: absolute;
  top: 100%;
  left: 0;
  z-index: 1000;
  display: none;
  float: left;
  min-width: 10rem;
  padding: .5rem 0;
  margin: .125rem 0 0;
  font-size: 1rem;
  color: #212529;
  text-align: left;
  list-style: none;
  background-color: #fff;
  background-clip: padding-box;
  border: 1px solid #c8ced3;
  border-radius: .25rem;
}

.dropdown-menu.show {
  display: block;
}

.dropdown-item {
  display: block;
  width: 100%;
  padding: .25rem 1.5rem;
  clear: both;
  font-weight: 400;
  color: #212529;
  text-align: inherit;
  white-space: nowrap;
  background-color: transparent;
  border: 0;
}

.dropdown-item:focus, .dropdown-item:hover {
  color: #16181b;
  text-decoration: none;
  background-color: #f8f9fa;
}

JS部分：

$('.dropdown-toggle').on('click', function() {
  $('.dropdown-menu').toggleClass('show');
});

$('.dropdown-item').on('click', function() {
  $('.dropdown-toggle').text($(this).text());
  $('.dropdown-menu').removeClass('show');
});

通过以上代码，可以实现一个基本的HTML+CSS+JS下拉菜单。当点击下拉菜单按钮时，JS代码会添加或移除下拉菜单的show类，实现下拉菜单的显示或隐藏；当点击下拉菜单的选项时，JS代码会将选项的文本内容设置为下拉菜单按钮的文本内容，并移除下拉菜单的show类，实现选项的选择和下拉菜单的隐藏。