css flex box
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>css flexbox</title>
<style>
*{
margin: 0;
padding: 0;
}
.main{
height: 100vh;
width: 100vw;
display: flex;
justify-content: center;
align-items: center;
}
.greenBox{
height: 200px;
width: 200px;
background-color: rgb(21, 228, 21);
}
</style>
</head>
<body>
<div class="main">
<div class="greenBox"></div>
</div>
</body>
</html>