1、首先打开电脑检查网络。
2、其次打开进行测试网络连接是否异常。
3、最后点击重新进入即可。
<!DOCTYPE HTML><html>
<body>
<details>
<summary>Copyright 2011.</summary>
<p>All pages and graphics on this web site are the property of W3School.</p>
</details>
</body>
</html>
参考以下代码：
<!DOCTYPE html><html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
* {
margin: 0
padding: 0
}
#parent >li> span{background: #999999display: blockwidth: 200pxborder:1px solid #ECEEF2}
li {line-height: 40pxdisplay: block}
li p{
display: inline-block
width: 0px
height: 0px
border-left: 5px solid transparent
border-right: 5px solid transparent
border-top: 5px solid#2f2f2f
}
li>ul{display: none}
li>ul>li{border: 1px solid #DEDEDEwidth: 199px}
#parent span:hover + ul{display: block}
#parent span:hover >p{
display: inline-block
width: 0px
height: 0px
border-top: 5px solid transparent
border-bottom: 5px solid transparent
border-left: 5px solid#2f2f2f}
</style>
</head>
<body>
<ul id="parent">
<li>
<span><p></p>列表</span>
<ul>
<li>子列表</li>
<li>子列表</li>
<li>子列表</li>
</ul>
</li>
<li>
<span><p></p>列表</span>
<ul>
<li>子列表</li>
<li>子列表</li>
<li>子列表</li>
</ul>
</li>
<li>
<span><p></p>列表</span>
<ul>
<li>子列表</li>
<li>子列表</li>
<li>子列表</li>
</ul>
</li>
</ul>
</body>
</html>
