Of all the HTML guides I've seen, John December has the best. Particularly useful are the "Hex Hub" in his "Color Spot" section and the guides for the latest transitional and strict tags in his "HTML Station".
December's definitely quirky--see his "Live Simple" ebook, which apparently aspires to be a guide to everything from organizing your home to choosing an ISP to "living your dream"--but it's the sort of hyper-organized approach to life you'd expect from the guy who put together the superb resources above.
