As the web and its underlying technologies evolve fundamentals change as well. Light years ahead of the early days of the web when a developer needed a desk side paperweight reference just to get by, HTML5, though not perfect, has finally been accepted as the coveted WC3 Recommendation. The light is finally showing through to the world.
Although we are not going to cover an extensive onset of XHTML -vs- HTML5 comparisons, there is an interesting article by our friends at sitepoint outlining Five Obsolete Features of HTML5. A good read for the practicing coder. Enjoy.