You can set up all of your notifications to be sent in HTML format or as plain text (email notification, autoresponder, copy of the submission, payment notification). To send notifications in HTML format or as plain text, simply edit or create a template and select one of the two radio buttons on top: HTML or Plain Text.
If you want to set up a message in HTML format for an email notification, autoresponder or for the copy the the submission to the sender, go to the Settings → Notifications → Email Notifications section of your form, select one of the notification options and hit Customize. Select the HTML option, on top, to send the message as HTML.
To send payment notifications in HTML format, go to the Settings → Payments section of your form, hit Customize message, under Notifications, and tick the HTML option.