使用 CSS 可以创建一个鼠标移入后显示下拉菜单的效果。
1、下拉菜单的实现
当鼠标移入指定元素时，显示下拉菜单。代码如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>简单的下拉菜单</title>
<style>
.dropdown{
position:relative;
display:inline-block;
}
.dropdown-content{
min-width:200px;
border:1px solid black;
position:absolute;
display:none;
}
.dropdown:hover .dropdown-content{
display:block;
}
</style>
</head>
<body>
<p class="dropdown">
<span>鼠标你过来，我为你展示下拉菜单。</span>
<p class="dropdown-content">
<p>下拉菜单 1</p>
<p>下拉菜单 2</p>
</p>
</p>
</body>
</html>实例解析：
HTML 部分：
可以使用任何 HTML 元素来打开下拉菜单，比如 45a2772a6b6107b401db3c9b82c049c2，或 3499910bf9dac5ae3c52d5ede7383485、bb9345e55eb71822850ff156dfde57c8 等元素。
然后，使用 e388a4556c0f65e1904146cc1a846bee 元素来创建一个容器，用于包含下拉菜单的内容，并自定义他需要出现的位置。
最后，再使用 e388a4556c0f65e1904146cc1a846bee 元素来包裹这些元素，并使用 CSS 来设置下拉内容的样式。
CSS 部分：
.dropdown 类使用 position:relative，这将设置下拉菜单的内容放置在下拉按钮（使用 position:absolute）的右下角位置。
.dropdown-content 类中是实际的下拉菜单。默认是隐藏的，在鼠标移入指定元素后显示。
注意：在上面的例子中，下拉菜单容器的宽度设置为最小 200px，根据实际情况可以自行定义。
如果希望下拉菜单与下拉按钮的宽度一致，可将宽度设置为 100%，overflow:auto 属性可以指定在小尺寸屏幕上滚动。
:hover 选择器用于在用户将鼠标移动到下拉按钮上时显示下拉菜单，将下拉菜单容器设置为 display:block;
2、创建下拉菜单
在上面例子的基础上，创建一个下拉菜单，并允许用户选取列表中的某一项目。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>下拉菜单实例</title>
<style>
/* 容器 <p> - 需要定位下拉内容 */
.dropdown{
position:relative;
display:inline-block;
}
/* 下拉菜单内容，设置为隐藏 */
.dropdown-content{
min-width:150px;
background-color:#F5F5F5;
position:absolute;
display:none;
box-shadow:0px 8px 15px 0px rgba(0,0,0,0.2);
}
/* 下拉按钮样式 */
.dropbtn{
color:white;
font-size:16px;
padding:15px;
border:none;
background-color:#00CD66;
cursor:pointer;
}
/* 下拉菜单链接的样式 */
.dropdown-content a{
color:black;
text-decoration:none;
padding:12px 15px;
display:block;
}
/* 鼠标移入后显示下拉菜单 */
.dropdown:hover .dropdown-content{
display:block;
}
/* 鼠标移入后修改链接的背景色 */
.dropdown-content a:hover{
background-color:#E8E8E8;
}
/* 下拉菜单显示后修改下拉按钮的背景色 */
.dropdown:hover .dropbtn{
background-color:#008B00;
}
</style>
</head>
<body>
<p class="dropdown">
<button type="button" class="dropbtn">下拉菜单按钮</button>
<p class="dropdown-content">
<a href="#">下拉菜单项 1</a>
<a href="#">下拉菜单项 2</a>
<a href="#">下拉菜单项 3</a>
</p>
</p>
</body>
</html>在 Chrome 中当鼠标移入下拉菜单按钮时显示如下：
在上面的例子中，下拉菜单的内容设置了 box-shadow 属性，这个属性主要用于添加阴影，这样下拉菜单看起来就像一个“卡片”一样。该属性指定下拉菜单在水平方向阴影为 0，垂直方向为 8 像素的阴影，模糊距离为 15 像素，阴影的尺寸为 0，并指定阴影的颜色为黑色，且透明度为 0.2。
下拉菜单按钮使用了 cursor 属性，该属性主要用于指定鼠标光标的类型，即鼠标指针放在一个元素边界范围内时所用的光标形状。
下表列出了该属性所有的值： 鼠标可移入对应的属性值，查看光标的形状。
| 值 | 说明 |
| URL | 需要使用的自定义光标的 URL。注意：在此列表的末端始终定义一种普通的光标，以防没有由 URL 定义的可用光标。 |
| default | 默认光标，通常是一个箭头。 |
| auto | 默认，浏览器设置的光标。 |
| pointer | 光标显示为指示链接的指针，即一只手。 |
| text | 此光标指示文本。 |
| wait | 此光标指示程序正忙，显示为一个转动的蓝色小圆圈。 |
| help | 此光标指示可用的帮助，显示为一个箭头右下角带有问号。 |
| crosshair | 光标显示为十字线。 |
| move | 此光标指示某对象可被移动。 |
| e-resize | 此光标指示矩形框的边缘可被向右（东）移动。 |
| ne-resize | 此光标指示矩形框的边缘可被向上及向右移动（北/东）。 |
| nw-resize | 此光标指示矩形框的边缘可被向上及向左移动（北/西）。 |
| n-resize | 此光标指示矩形框的边缘可被向上（北）移动。 |
| se-resize | 此光标指示矩形框的边缘可被向下及向右移动（南/东）。 |
| sw-resize | 此光标指示矩形框的边缘可被向下及向左移动（南/西）。 |
| s-resize | 此光标指示矩形框的边缘可被向下移动（南）。 |
| w-resize | 此光标指示矩形框的边缘可被向左移动（西）。 |
3、下拉菜单的对齐方式
如果下拉菜单设置了 float:left 样式，那么还需要设置 left:0，即从 left:0 处开始从左向右显示菜单内容，否则会超出浏览器左边窗口。
如果下拉菜单设置了 float:right 样式，默认都是从左向右显示，菜单内容则会超出浏览器右边窗口，所以还需要设置 right:0，让下拉菜单内容从右向左。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>下拉菜单对齐</title>
<style>
/* 容器 <p> - 需要定位下拉内容 */
.dropdown{
position:relative;
display:inline-block;
}
/* 下拉菜单内容，设置为隐藏 */
.dropdown-content{
min-width:150px;
background-color:#F5F5F5;
position:absolute;
right:0;
display:none;
box-shadow:0px 8px 15px 0px rgba(0,0,0,0.2);
}
/* 下拉按钮样式 */
.dropbtn{
color:white;
font-size:16px;
padding:15px;
border:none;
background-color:#00CD66;
cursor:pointer;
}
/* 下拉菜单链接的样式 */
.dropdown-content a{
color:black;
text-decoration:none;
padding:12px 15px;
display:block;
}
/* 鼠标移入后显示下拉菜单 */
.dropdown:hover .dropdown-content{
display:block;
}
/* 鼠标移入后修改链接的背景色 */
.dropdown-content a:hover{
background-color:#E8E8E8;
}
/* 下拉菜单显示后修改下拉按钮的背景色 */
.dropdown:hover .dropbtn{
background-color:#008B00;
}
</style>
</head>
<body>
<p class="dropdown" style="float:left;">
<button type="button" class="dropbtn">左</button>
<p class="dropdown-content" style="left:0;">
<a href="#">下拉菜单项 1</a>
<a href="#">下拉菜单项 2</a>
<a href="#">下拉菜单项 3</a>
</p>
</p>
<p class="dropdown" style="float:right;">
<button type="button" class="dropbtn">右</button>
<p class="dropdown-content">
<a href="#">下拉菜单项 1</a>
<a href="#">下拉菜单项 2</a>
<a href="#">下拉菜单项 3</a>
</p>
</p>
</body>
</html>在 Chrome 中当鼠标移入右下拉按钮时显示如下：
4、下拉图片
下的例子，在下拉菜单中添加图片：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>下拉图片</title>
<style>
/* 容器 <p> - 需要定位下拉内容 */
.dropdown{
position:relative;
display:inline-block;
}
/* 下拉菜单内容，设置为隐藏 */
.dropdown-content{
min-width:300px;
background-color:#F5F5F5;
position:absolute;
display:none;
box-shadow:0px 8px 15px 0px rgba(0,0,0,0.2);
}
/* 鼠标移入后显示下拉菜单 */
.dropdown:hover .dropdown-content{
display:block;
}
/* 图片文字描述的样式 */
.heading{
padding:15px;
text-align:center;
}
</style>
</head>
<body>
<p class="dropdown">
<img src="images/1.jpg" width="120" height="120" alt="">
<p class="dropdown-content">
<img src="images/xi.jpg" width="300" height="200" alt="">
<p class="heading">最美的时光，留给最美的风景！</p>
</p>
</p>
</body>
</html>在 Chrome 中当鼠标移入图片下拉按钮时显示如下：
5、导航栏下拉菜单
下面的例子，在导航栏菜单中添加下拉菜单：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>导航栏下拉菜单</title>
<style>
*{margin:0;padding:0;}
ul{
list-style-type:none;
background-color:#333;
overflow:hidden;
}
li{
float:left;
}
li a, .dropbtn{
display:inline-block;
color:white;
text-align:center;
padding:14px 16px;
text-decoration:none;
}
li a:hover, .dropdown:hover .dropbtn{
background-color:#111;
}
.dropdown{
display:inline-block;
}
.dropdown-content{
min-width:200px;
background-color:#F9F9F9;
position:absolute;
display:none;
box-shadow:0px 8px 15px 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;
}
.dropdown:hover .dropdown-content{
display: block;
}
</style>
</head>
<body>
<ul>
<li><a href="#shop" class="active">在线商城</a></li>
<li><a href="#news">产品展示</a></li>
<p class="dropdown">
<a href="#" class="dropbtn">下拉菜单</a>
<p class="dropdown-content">
<a href="#">子菜单 1</a>
<a href="#">子菜单 2</a>
<a href="#">子菜单 3</a>
</p>
</p>
</ul>
</body>
</html>在 Chrome 中当鼠标移入下拉按钮时显示如下：
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