You use cascading style sheets to control the layout and appearance of your topic text and elements. These style sheets allow you to define a full array of layout specifications for HTML topics such as font style and size, line spacing, paragraph indents, and more. Similar to templates used by Microsoft Word and other programs, a style sheet is a template that controls the formatting of HTML Help topics. You can alter the appearance of an HTML Help topic by changing the formatting assigned to it from a style sheet.
There are three ways to work with style sheets and styles:
External style sheets: Style definitions are stored in a style sheet file (.CSS) that is separate from the HTML topics that use it. A single style sheet can be linked to any number of HTML topics. The individual styles that are defined in the style sheet are available to all topics that use the style sheet. When you modify a style, all topics that are linked to the style sheet are automatically updated.
Embedded styles: Style definitions are embedded (saved) in an individual HTML Help file. If the file is linked to a style sheet, all embedded styles override the styles in the style sheet that are applied to the text.
Inline styles: A block of text within a single HTML Help file is formatted directly in the WYSIWYG Editor. If the file is linked to a style sheet, all inline styles override styles in the style sheet and any embedded styles that may have been saved within the file as well.
Cascading style sheets is the term for HTML styles developed by the World Wide Web Consortium (W3C). "Cascading" refers to the hierarchy of the three styles (external style sheets, embedded, inline), when they are used in combination.