Here’s a quick HTML template you can use to get started with jQuery UI. It uses Google Libraries API and already accounts for jQuery UI font sizes.
<!DOCTYPE html>
<html>
<head>
<link type="text/css" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/themes/base/jquery-ui.css" rel="stylesheet" />
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/jquery-ui.min.js"></script>
<style>
body { font-size: 62.5%; }
</style>
<script>
$(function() {
$( "#dialog" ).dialog();
});
</script>
</head>
<body>
<div id="dialog" title="Dialog">
<p>Hello World!</p>
</div>
</body>
</html>
Here’s what it looks like in the browser.
