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Tools

From MDC

Web Development

Extensions

Venkman
JavaScript debugger provides a powerful JavaScript debugging environment for Mozilla based browsers.
DOM Inspector
A developer tool used to inspect, browse, and edit the DOM of documents - usually web pages or XUL windows.
Firebug
Integrates a wealth of development tools to edit, debug, and monitor CSS, HTML, and JavaScript live in any web page (details)
Web Developer
Adds a menu and a toolbar to the browser with various web developer tools. (details)
Aardvark
Reveals DOM elements as you scrub the mouse over a page. (details)
More on Firefox Add-ons

Validators

List of validators

Authoring Software

Standards-Compliant Authoring Tools
HTMLTidy
A tool for cleaning HTML

JavaScript

Error Console
Venkman
JSLint
JSDoc
tool for generation of (usually HTML) documentation from code comments (similar to JavaDoc).

DOM

DOM Inspector
MODI
Mouseover DOM Inpector is a bookmarklet that allows you to view and manipulate the DOM of a web page simply by mousing around the document.

Localization

Localization and Leveraging Tools

Tree and branch management

cross-commit script