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.
The best advice I can give you is to check out some of the “best of CSS” websites that rank top CSS & XHTML sites. Go to these sites and view their page sources, then see why and how the page was written the way it is. Then you can truly understand what the experts are doing (by way of example), and you can replicate these ideas in whatever project you are working on. The possibilities with these 2 web languages are endless, and with the addition of javascript you can make a really nice website that will keep your visitors entertained for much longer.