XHTML 1.1 is a cleaner form of HTML 4.0 which was last used in 1999. XHTML has seperated the page content from the styles used on the pages by using CSS. This allows designers to change styles more rapidly when they are needed and it makes the coding of the pages easier to read for humans and easier to parse thru for search engines.

XHTML follows standards that are developed by W3C. Validation of pages that are produced for the web is important so that the page is displayed properly in various browsers. Visit their website to learn more about XHTML Validation and CSS Validation.