忘记给出代码了:在这里补上

<style>

.container {
  width: 280px;
  margin: 0 auto;
  padding: 40px 0;
  background-color: #f0f3f9;
  animation: width 2s infinite alternate;
}

@keyframes width {
  from { width: 200px; }
  to { width: 280px; }
}

.textarea {
  padding: 9px 8px;
  border: 1px solid #d0d0d5;
  border-radius: 4px;
  background-color: #fff;  
}

.textarea > textarea {
  width: 100%;
  line-height: 20px;
  padding: 0;
  border: 0 none;
  outline: 0 none;
  background: none;
  resize: none;
}
</style>
<div class="container">
  <div class="textarea">
    <textarea rows="5" placeholder="这里是代码：<style>
.container {
  width: 280px;
  margin: 0 auto;
  padding: 40px 0;
  background-color: #f0f3f9;
  animation: width 2s infinite alternate;
}

@keyframes width {
  from { width: 200px; }
  to { width: 280px; }
}

.textarea {
  padding: 9px 8px;
  border: 1px solid #d0d0d5;
  border-radius: 4px;
  background-color: #fff;  
}

.textarea > textarea {
  width: 100%;
  line-height: 20px;
  padding: 0;
  border: 0 none;
  outline: 0 none;
  background: none;
  resize: none;
}
</style>
<div class="container">

  <div class="textarea">

    <textarea rows="5" placeholder="文本内容，这里可以输入文本"><textarea>
  <div>
<div>