id

Этот атрибут определяет уникальное имя элемента.

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div id="div1">
<p>Hello World!</p>
</div>
<script>
$(document).ready(function () {
$("*").css("margin", "0").css("padding", "0");
$("#div1").css({ "width": "500px", "height": "500px", "position": "absolute", "left": "50%", "top": "50%", "margin-left": "-250px", "margin-top": "-250px", "text-align": "center", "line-height": "500px", "background-color": "#EEFCCD", "font-size": "50px" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div id="div1">
<p>Hello World!</p>
</div>
<script>
$(document).ready(function () {
$("*").css("margin", "0").css("padding", "0");
$("#div1").css({ "width": "500px", "height": "500px", "position": "absolute", "left": "50%", "top": "50%", "margin-left": "-250px", "margin-top": "-250px", "text-align": "center", "line-height": "500px", "background-color": "#EEFCCD", "font-size": "50px" });
});
</script>
</body>
</html>