The document discusses various CSS pseudo-classes and pseudo-elements that can be used to style elements in different states or add styled content before or after elements. It provides examples of how to use the :link, :visited, :hover, :active, :focus, :first-child, :lang, :first-letter, :first-line, :before, and :after pseudo-classes and pseudo-elements to style links, first children, elements in different languages, and add content.