Content, Structure, and Presentation - Content is a collective term for all the text, images, videos, sounds, animations, and files that you want to show your audience. XHTML enables you to define what each element of your content is. CSS allows you to define how each element looks and presents to your audience.
Times are Changing - Web standards are now widely adopted which allows designers to code their pages with clean structure. Before the standards everything had to be coded as tables in order to layout their page. Today the majority of browsers use XHTML and CSS as a standard.
How to write XHTML - CSS is a mechanism used to stylize HTML, so you can’t use a CSS file without a XHTML file. XHMTL is based on free-form XML which allows tags to be named to describe the content they contain. By becoming XML compliant XHTML has been able to transcend the limitations of HTML and will be able expand over time.