With the recent explosion of mobile devices on the web, and the way people consume content on websites, it's become increasingly more important to have pages written in valid xhtml, with a clear separation of content and design (css). The trick for designers is to create a page that displays well both on the screen, in different browsers, on mobile devices, and in print as well. Additionally, so long as the structure is written well without unnecessary div wrappers or *gasp* tables, a well coded page will display just fine on a text only browser.