Even if the document links to an external style sheet, new style declarations are written to the head of the document, not in the CSS file.
This general behavior may differ in the following situations:
If the document already formats everything using font tags, Dreamweaver uses font tags and modifies the body tag to use HTML code.
If the document’s body tag uses HTML to set the page’s appearance, but the page doesn’t exclusively use font tags, Dreamweaver still uses CSS to format text.
If you deselect this option, Dreamweaver uses HTML tags such as font tags to format text, and HTML code in the body tag to set the page’s appearance.