Browser Tip of the Day: HTML, CSS, and DOM reference right at your fingertips, plus a few more
DevBoi is a simple and cool extension with a silly name by Martin Cohen. It's a sidebar for Firefox that gives you quick access to the documentation for HTML (plus entities), CSS, DOM, XUL, Ruby, and Ruby on Rails. All that's lacking is a modern XHTML 1.0 reference (and adding PHP, Java, Python, JavaScript, and so on wouldn't hurt), and perhaps an about:config-style filter box. Hopefully those will be included in a future release.
