During a makeover of a site I ran into a great site which puts up what that has been removed from strict xhtml and also a very good overview of all elements and attributes for different doctypes. You can search or browse all elements and see which attributes that are allowed which is of great help when you’re cleaning up a site that uses old attributes. Great example is ”accesskey” which is still allowed in href, labels and a few more, but not in header elements and so forth. Go over there and make sure to bookmark it!
*edit* Thanks Jonatan, what a misstake I made 🙂 the links are now correct