CSS (Cascading Style Sheets) allow separation of content and presentation through chained classes. Chained classes involve assigning multiple classes to HTML elements, separated by spaces. In CSS, these classes can then be concatenated to target elements with all classes. This allows common styling as well as customized styling based on class combinations. Examples show using chained classes for messages, site sections, and dynamic styling of specific pages. Chained classes help achieve real separation of concerns between HTML and CSS.