JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css">
<div class="container-fluid">
<div class="span5 well">
<h2>Authentification</h2>
<form method="POST">...</form>
</div>
</div>

CSS

.container-fluid {
    height:500px;
    border:1px solid #000;

   
}
div.span5 {
    margin: 0 auto;
    top:50%;
    width:400px;
    position:relative;
    transform: translateY(-50%);
}