外层div随内层div高度自适应

首先说一下textarea的高度随文字的内容自适应，用div模拟textarea。直接看代码。其中

contenteditable="true"表示div可以编辑。。主要是设置

overflow: auto;

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style type="text/css">
    .textarea{
      width: 300px;
      border:1px solid #ccc;
      min-height:30px;
      overflow: auto;
      outline:0;
     }
 </style>
</head>
<body>

  <div class="textarea" contenteditable="true">textarea</div>

</body>
</html>

再说下外层的div随内层的div高度自适应。直接看代码。起到作用的是在外层div的样式上加display:table

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style type="text/css">
  .box{
      width: 800px;height: 400px;
      border: 1px solid #000;
      position: relative;
      display: table;
    }
    .textarea{
      width: 300px;
      border:1px solid #ccc;
      min-height:30px;
      overflow: auto;
      outline:0;
     }
 </style>
</head>
<body>
<div class="box">
  <div class="textarea" contenteditable="true">textarea</div>
</div>
</body>
</html>