无论是内联元素，还是原本就是块级元素，都会变成块级元素，同时这个元素会建立新的块级格式上下文
- 清除浮动
- 去除 margin 合并现象
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>display: flow-root</title>
<style>
p {
outline: solid deepskyblue;
}
img {
float: left;
width: 128px;
}
.flow-root {
display: flow-root;
}
.box {
background-color: #f0f3f9;
}
.box p {
margin: 2em;
}
</style>
</head>
<body>
<h4>1. 去除浮动</h4>
<p><img src="https://image.zhangxinxu.com/image/study/s/s256/mm.jpg" /></p>
<div style="clear: both"></div>
<p class="flow-root">
<img src="https://image.zhangxinxu.com/image/study/s/s256/mm.jpg" />
</p>
<h4>2. 去除margin合并</h4>
<div class="box">
<p>margin: 2em;</p>
</div>
<div class="box flow-root">
<p>margin: 2em;</p>
</div>
</body>
</html>
