HTML is the backbone to every website, it is the head, the shoulders, and the feet of the website. We used to design simple websites that had several graphics slapped on them. These days, using only html on your website or blog can mean that you arent using all the tools in the shed to get your website up to par.
Using XHTML & CSS together means you can do wonderful things with images and fonts that you never thought before. If you visit websites like csszengarden, you will see hundreds of CSS & XHTML pages that were created and submitted by web designers. Looking at several of these pages can open your eyes to see that using only HTML on a site is a very big setback.
If you are interested in creating CSS & XHTML websites and you want guidance, then you can use the w3c validation pages, where you can simple plug in your page url and have the validation tool check to see if your website is using proper XHTML and CSS structures.