The HTML Playground offers XHTML and CSS references by example. The site is still in beta at version 0.4, but offers an easy to use interface that makes digesting all the information displayed simple. It is a fast loading application and I have just found it to be overall a great tool for learning the basics of HTML and CSS. Read the full article
This site is one of those "brain-blank" incredibly useful sites - and I love the fact that it gives you not only inline sample usage but the display, plus options to test yourself.