html布局bug，为什么这个div标签始终不能放在另一个div标签里？
我试过改变标签display样式但是都没有效果，清除浮动也不行
源码如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>下拉菜单</title>
<style>
.menu {
height: 400px;
width: 1000px;
border: 2px solid skyblue;
margin-top: 100px;
margin-left: auto;
margin-right: auto;
display: flex;
}
.menu .head{
height: 100px;
}
.menu .title {
height: 400px;
width: 200px;
background-color: gold;
}
.title .content{
height: 300px;
display: inline-block;
background-color:#aaa;
}
a{
text-decoration: none;
}
.clearfix:after{
clear: both;
}
</style>
</head>
<body>
<div class="menu">
<div class="title clearfix">学习</div>
<div class="content">
<a href="https://nav.guidebook.top/xuexi" target="_blank">学习教育</a>
<a href="http://muchong.com/bbs/" target="_blank">小木虫论坛</a>
<a href="https://www.elecfans.com/" target="_blank">电子发烧友</a>
<a href="https://www.zhihuishu.com/" target="_blank">智慧树平台</a>
<a href="https://www.51zxw.net/" target="_blank">我要自学网</a>
<a href="https://cet-bm.neea.edu.cn/Index" target="_blank">英语四六级报名网</a>
</div>
<div class="title">编程</div>
<!-- <a href="https://www.code-nav.cn/" target="_blank">编程导航</a>
<a href="https://cplusplus.com/reference/" target="_blank">C语言头文件官网</a>
<a href="https://zh.cppreference.com/w/%E9%A6%96%E9%A1%B5" target="_blank">中文C语言官网</a>
<a href="https://ac.nowcoder.com/" target="_blank">牛客竞赛网</a>
<a href="https://leetcode.cn/" target="_blank">力扣</a>
<a href="https://www.matiji.net/exam/" target="_blank">码蹄集</a> -->
<div class="title">工具</div>
<!-- <a href="http://www.hippter.com/" target="_blank">PPT模板导航</a>
<a href="https://www.excelbbx.net/Word.htm" target="_blank">Word精灵</a>
<a href="https://www.toolhelper.cn/Digit/FractionConvert" target="_blank">计算工具大全</a>
<a href="https://developer.mozilla.org/zh-CN/docs/Web/HTML/Element" target="_blank">HTML大全</a>
<a href="https://cn.bing.com/translator/?h_text=msn_ctxt&setlang=zh-cn" target="_blank">bing翻译</a>
<a href="https://pyecharts.org/#/zh-cn/intro" target="_blank">pyecharts图表官网</a> -->
</div>
</body>
</html>
