父元素未设置高度和宽度的时候。那么它的高度就会塌缩为零
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
.box1{
border: 1px solid red;
background-color: red;
}
.box2{
width: 200px;
height: 200px;
background-color: pink;
float: left;
}
</style>
</head>
<body>
<div class="box1">
<div class="box2"></div>
</div>
</body>
</html><!DOCTYPE html
