Не пойму, почему не работает box-sizing:border-box

body {
  margin: 0;
}

h1 {
  font-family: 'Alata', sans-serif;
  font-size: 80px;
  margin: 0;
}

div.header {
  width: 900px;
  margin: 0 auto;
  text-align: center;
  padding-top: 400px;
}

div.full_page {
  width: 100%;
  height: 100vh;
  background: green;
}

div.intro {
  width: 900px;
  height: 900px;
  margin: 0 auto;
  background: red;
  box-sizing: border-box;
}

<link href="https://fonts.googleapis.com/css2?family=Alata&family=Baloo+Tamma+2&display=swap" rel="stylesheet">
<div class='full_page'>

  <div class='intro'>
    <div class="header">
      <h1>Design</h1>
    </div>
  </div>

</div>