W3School TIY Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
body {
  font-family: "Lato", sans-serif;
}

/* 固定的侧边导航栏，全高 */
.sidenav {
  height: 100%;
  width: 200px;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: #111;
  overflow-x: hidden;
  padding-top: 20px;
}

/* 设置侧边栏链接和下拉菜单按钮的样式 */
.sidenav a, .dropdown-btn {
  padding: 6px 8px 6px 16px;
  text-decoration: none;
  font-size: 20px;
  color: #818181;
  display: block;
  border: none;
  background: none;
  width: 100%;
  text-align: left;
  cursor: pointer;
  outline: none;
}

/* 鼠标悬停时 */
.sidenav a:hover, .dropdown-btn:hover {
  color: #f1f1f1;
}

/* 主内容 */
.main {
  margin-left: 200px; /* 与侧边栏的宽度相同 */
  font-size: 20px; /* 增大文本以启用滚动 */
  padding: 0px 10px;
}

/* 为活动的下拉菜单按钮添加一个活动类 */
.active {
  background-color: green;
  color: white;
}

/* 下拉菜单容器（默认为隐藏）。可选：为下拉菜单内容添加更亮的背景色和一些左内边距以更改其设计 */
.dropdown-container {
  display: none;
  background-color: #262626;
  padding-left: 8px;
}

/* 可选：设置向下箭头图标的样式 */
.fa-caret-down {
  float: right;
  padding-right: 8px;
}

/* 一些用于实现响应式设计的媒体查询 */
@media screen and (max-height: 450px) {
  .sidenav {padding-top: 15px;}
  .sidenav a {font-size: 18px;}
}
</style>
</head>
<body>

<div class="sidenav">
  <a href="#about">About</a>
  <a href="#services">Services</a>
  <a href="#clients">Clients</a>
  <a href="#contact">Contact</a>
  <button class="dropdown-btn">Dropdown 
    <i class="fa fa-caret-down"></i>
  </button>
  <div class="dropdown-container">
    <a href="#">Link 1</a>
    <a href="#">Link 2</a>
    <a href="#">Link 3</a>
  </div>
  <a href="#contact">Search</a>
</div>

<div class="main">
  <h2>侧边栏下拉菜单</h2>
  <p>一些随机文本..下一个示例将向您展示如何实现下拉菜单链接的功能。</p>
  <p>此侧边栏为全高（100%），且始终显示。</p>
  <p>一些随机文本..</p>
</div>

</body>
</html>