HTML Playground
HTML Playground is a website for testing xhtml and css. You select an html or css tag and modify any attributes that may be associated with it. HTML Playground gives you a description of the tag, and you can then see an example of how the tag is used — shown both as html code and how it is rendered. You can even modify the example and view the result. This is very useful for quick experimentation with html and css.
Visit HTML Playground [via gHacks]