Every HTML element in the source code generates a rectangular area in the layout of the resulting web page - known as its formatting box. The sum of all these areas determine the layout for the given page. The measurements of formatting boxes can be changed using CSS style rules.
With an activated Selection Mode in the web pane the formatting box of each selected HTML element is highlighted using a semi-transparent overlay. The margin is displayed using a pattern by default. The padding gets a darker shade of blue.