Style Side Panel
Inside the Style Side Panel you can see the CSS styles, that are currently assigned to the selected node inside the HTML Panel. It also offers you similar possibilities of CSS manipulation like the CSS Panel. The difference between them is organizational. The Style Side Panel is a live view of the styles assigned to the selected node inside the HTML Panel (which can lie in different files) while the CSS Panel shows all styles of a CSS containing file.
Furthermore the Style Side Panel gives you the cascading info, meaning styles, that are overwritten by other style definitions will be shown lined-through.