The order number for a completed order is an important piece of information to display to customers on the Thank You Page and E-Mail Receipt. It shows that the order has been entered into "the system" and gives the customer a way to refer to the order for any questions that might arise.
The E-Mail Receipt use the same tags shown here for the Thank You Page.
The output of the order number tags does not contain any CSS classes or attributes. You can wrap the tags in an HTML table cell or div or span to apply any additional formatting.
Next: Confirmation & Thank You Totals