class

Этот атрибут определяет класс.

style.css

* {
margin: 0;
padding: 0;
}
.a {
width: 400px;
height: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
background-color: #E2DDE3;
text-align: center;
line-height: 400px;
font-size: 30px;
}

* {
margin: 0;
padding: 0;
}
.a {
width: 400px;
height: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
background-color: #E2DDE3;
text-align: center;
line-height: 400px;
font-size: 30px;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="style.css" rel="stylesheet" />
</head>
<body>
<div class="a">
<p>Hello World!</p>
</div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="style.css" rel="stylesheet" />
</head>
<body>
<div class="a">
<p>Hello World!</p>
</div>
</body>
</html>