<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.box{
display: flex;
justify-content: center;
align-items: center;
width: 200px;
height: 200px;
background-color: #222;
}
.item{
width: 20px;
height: 20px;
background-color: #f00;
}
</style>
</head>
<div class="box">
<span class="item"></span>
</div>
</body>
</html>