W3School TIY Editor

<!DOCTYPE html>
<html>
<head>
<style>
.dropbtn {
  background-color: #4CAF50;
  color: white;
  padding: 16px;
  font-size: 16px;
  border: none;
  cursor: pointer;
}

.dropbtn:hover, .dropbtn:focus {
  background-color: #3e8e41;
}

.dropdown {
  position: relative;
  display: inline-block;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  overflow: auto;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropdown-content a:hover {background-color: #f1f1f1}

.show {display:block;}
</style>
</head>
<body>

<h2>onclick 事件</h2>

<p>请点击按钮来打开下拉菜单。</p>

<div class="dropdown">
  <button id="myBtn" class="dropbtn">Dropdown</button>
  <div id="myDropdown" class="dropdown-content">
    <a href="#home">Home</a>
    <a href="#about">About</a>
    <a href="#contact">Contact</a>
  </div>
</div>

/* myFunction 在添加和删除显示类之间切换，用于隐藏和显示下拉内容 */
function myFunction() {
  document.getElementById("myDropdown").classList.toggle("show");
}
</script>

</body>
</html>