Firebug

Firebug is a Web development tool that facilitates the debugging, editing, and monitoring of any Web site's CSS, HTML, DOM, XHR, and JavaScript; it also provides other web development tools. Firebug's JavaScript panel can log errors, profile function calls, and enable the developer to run arbitrary JavaScript. Its net panel can monitor URLs that the browser requests, such as external CSS, JavaScript, and image files. The net panel can display both request headers and response headers for each page asset; it can also estimate the time each asset took to load.

In inspection mode, Firebug allows you to identify specific page (DOM) elements and get a listing of their class or id names and actual CSS code. This inspection tool is a great aid to finding specific CSS elements for making styling and modification changes.

Firebug has two major implementations: an extension (add-on) for Mozilla Firefox and a bookmarklet implementation called Firebug Lite. A cross-browser version is in development.

There are similar, but less capable tools, for other browsers.