Obviously, the recipient must be able to accept html emails. Here is what I do for MIME emails of the html type. The bold part is probably all you're looking for.
$message = "
<html>
<head>
<title>afds</title>
.....";
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";
$headers .= "From: $fromname <$from>\r\n";
$headers .= "To: $to\r\n";
$headers .= "Reply-To: $replytoname <$replyto>\r\n";
mail($to, $subject", $message, $headers);
You get the picture. Hope this helps.