CSS / XHTML
These days a lot of the methods we used when I first started making web pages are deprecated.
Using CSS in well-formed HTML is not only good practice in terms of bandwidth but also makes webpages that are simpler and less cluttered because all the formatting is done in a separate document. Making webpages in this way not only makes the designers life easier, using well-formed HTML pages using CSS also means that the pages are easier for search engine robots to crawl. Thus, make your pages in a friendly manner and everyone is happy.
Cross-platform HTML and CSS is essential for formatting modern webpages and can be used in conjunction with AJAX, javascript and DOM.
See books such as CSS: The Definitive Guide and get more great CSS and XHTML info at Usability Links
