Perfectamente centrado

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>Perfectly centered</title>
  <style>
/*  http://howtocenterincss.com/ */
    .mainDiv{
      width:500px;
      height:300px;
      position:fixed;
      top:50%;
      left:50%;
      transform:translate(-50%,-50%);
      background-color:pink;
      display:flex;
      justify-content:center;
      align-items:center;
    }
  </style>
</head>
<body>

  <div class="mainDiv">Centered</div>

</body>
</html>