I strongly recommend Appearance (CSS). That option will cause Dreamweaver to create CSS styles that correspond to the Page Properties settings, which is the recommended way to define page properties today. To see how this works, create a new blank page, set the page properties with the CSS option selected, and then you can study the styles DW automatically creates by looking for them in the CSS Styles Panel.

If you choose Appearance (HTML) DW uses the HTML attributes, which are no longer recommended.

Janine