HTML/CSS/Box Model/border bottom width
Set border-bottom-width to 2px
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
h2 {
border-bottom-width: 2px;
}
</style>
</head>
<body>
<h2>This is the title</h2>
<p>This is a text.</p>
</body>
</html>