font-weight

значения свойства:

css/style.css

.a {
 font-weight: normal;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<p class="a">Hello World!</p>
<br />
<p id="p1">Hello World!</p>
<br />
<p class ="b">Hello World!</p>
<script>
/*JavaScript*/
document.getElementById("p1").style.fontWeight = "bold";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "font-weight": "900" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<p class="a">Hello World!</p>
<br />
<p id="p1">Hello World!</p>
<br />
<p class ="b">Hello World!</p>
<script>
/*JavaScript*/
document.getElementById("p1").style.fontWeight = "bold";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "font-weight": "900" });
});
</script>
</body>
</html>