Explanation
To underline text using CSS, specify underline for text-decoration property.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=shift_jis">
<title>Sample</title>
<style type="text/css"><!--
.importText {
text-decoration:underline;
}
--></style>
</head>
<body>
<h1>Sample</h1>
<p>
This is the very <span class="importText">important point</span>.
</p>
</body>
</html>