Custom checkout fields allow the merchant to insert his own form fields into the Shipping Page form. There can be any number of form fields with associated HTML tags and text, but you don't need to worry about that for your template. The fields can be grouped and placed in five locations on the Shipping Page, and you refer to them in your template by the location number. Although these locations are identified fairly precisely by their field labels in the back office, you can place them anywhere you like in your template. You cannot control the content of the fields, but you can control the location.
The [-- SHIP_CustomHTML --] tags produce the contents of the merchant-defined custom checkout fields exactly as entered by the merchant. Therefore, there are no pre-defined CSS classes or attributes associated with these tags.
Next: Checkout Buttons