I've actually been asked this question several times lately, where someone wants to either brush up on their little used and mostly forgotten skills, or is looking to learn from scratch. The answer to the question is simple: XHTML all the way, and here are my reasons:

Those wonderful people in the Drupal community have been awesome citizens and have stepped up to the plate after the Drupal documentation team opened the doors for anyone to contribute to the official online "handbook" documentation.