<!DOCTYPE html>
<html lang="ja">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
  </head>
  <body>
    <div id="app">
      <input v-model="msg" placeholder="入力すると……">
      <p>{{msg}}</p>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/vue"></script>
  </body>
</html>
*,*::before,*::after{
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

body{
  margin: 30px;
  text-align: center;
}

input{
  padding: 15px;
  width: 300px;
  margin-bottom: 15px;
  border: 0;
	border-bottom: 1px solid #333;
  transition: all 1s;
}

input:focus{
  outline: none;
  border-bottom: 1px solid red;
}

p{
  font-family: serif;
  font-size: 32px;
  color: red;
}
let vm = new Vue({
  el: '#app',
  data:{
    msg:''
  }
})

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.