ViewPort :

.box1 {
width: 200px;
height: 200px;
background-color: red;
}

.container {
display: flex;
justify-content: center;
align-items: center;
height: 100vh;
width: 100vw;
}
(/style)

(meta charset="UTF-8")
(meta http-equiv="X-UA-Compatible" content="IE=edge")
(meta name="viewport" content="width=device-width, initial-scale=1.0")
(title> Document
(/title)
(/head)
(body)
(section class="container")
(div class="box1")
(/div)
(/section)
(/body)
(/html)
(br> more