p {counter-increment: paragraphNumber;}
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>CSS Example</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<style type='text/css'>
p:before {content: counter(paragraphNumber, upper-roman) ": ";}
h1 {counter-reset: paragraphNumber;}
p {counter-increment: paragraphNumber;}
</style>
</head>
<body>
<h1>Heading One</h1>
<p>Here is a paragraph.</p>
<p>Here is a paragraph.</p>
<h1>Heading Two</h1>
<p>Here is a paragraph.</p>
<p>Here is a paragraph.</p>
<h1>Heading Three</h1>
<p>Here is a paragraph.</p>
<p>Here is a paragraph.</p>
</body>
</html>