<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style type="text/css"> .outer{ border: red solid 2px; /* overflow:hidden;*/ } /* .outer:after{ content: ''; display: block; clear: both; }*/ .inner{ background:green; width: 200px; height: 200px; float: left; } </style>
</head>
<body>
<div class="outer">
  <div class="inner"></div>
</div>
</body>
</html>

https://codepen.io/bb798sky/pen/LwBLzN

在浮动元素的父元素上加上伪类，解决高度塌陷的问题

.outer:after{
      content: '';
      display: block;
      clear: both;
      }

在浮动元素的父元素上加上overfolow:hidden;形成BFC也可以解决。