<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>…</title>
<style>
html, body, .wrapper {
height: 100%;
}
.wrapper {
display: flex;
}
.centered {
width: 50%;
height: 50%;
margin: auto;
background-color: #3fc;
}
</style>
</head>
<body>
<div class="wrapper">
<div class="centered">Centered</div>
</div>
</body>
</html>