<div class="wrapper">

<h1>Center an item</h1>

  <p>The div below has a height of 300 pixels. The image is in the centre of that area horizontally and vertically.</p>
  
  <div class="box">
    <img alt="balloon" src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/12005/square-balloon.jpg">
  </div>

</div>

.box {
  padding: 1em;
  border: 4px solid #3F383C;
  border-radius: 4px;
  height: 300px;
  display: flex;
  align-items: center;
  justify-content: center;
}

body {
  background-color: #fff;
  color: #333;
  font: 20px Helvetica, Arial, sans-serif;
}

.wrapper {
  margin: 0 auto;
  max-width: 900px;
}