Allineamento verticale

<!DOCTYPE html>
<html>

<head>
  <style>
    #container {
      color: #fff;
      height: 400px;
      display: table-cell;
      vertical-align: middle;
      width: 600px;
      background: #00d;
    }
    
    #content {
      color: #fff;
      width: 50%;
      margin: auto;
      background: #d00;
    }
  </style>
</head>

<body>
  <div class="red" id="container">
    <div class="blue" id="content">
      this
      <br/> is
      <br/> a
      <br/> block
      <br/>

    </div>
  </div>
</body>

</html>