CodeBurner is a suite of tools for web developers, that provides reference material for HTML and CSS, integrated with a range of popular development environments.
CodeBurner for Opera provides reference information and code examples for HTML and CSS, in an easy-to-use Opera Widget. It's a widgetized version of the original CodeBurner for Firebug, but with a standalone interface, and more dedicated search tools.
CodeBurner provides tools for accessing the information in different ways. There's a text-based search that matches element, attribute and property names as you type, and a DOM inspector for looking up objects that appear in the source code of any page.
There's also a CSS inspector for looking through the stylesheets associated with the DOM inspector's page, giving you instant information about properties, selectors and at-rules.