https://www.opera.com/products/dragonfly
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about:
Welcome to Opera Dragonfly alpha, the foundations of Opera's upcoming Developer Tools. Debug JavaScript, inspect CSS and the DOM, and view any errors – Opera Dragonfly makes developing using Opera easier than ever, both on your computer and mobile phone.
http://dev.opera.com/articles/view/introduction-to-opera-dragonfly/
This link is now broken.
about:
We have unleashed the Dragonfly! Opera Dragonfly is Opera's all-new set of developer tools, designed to give developers a lightweight-but-powerful application that provides effective mechanisms for web standards debugging and problem solving without slowing down the browser, and fits in nicely with the development workflow.
