opera developer webdeveloper tools inspector DOM css javascript firebug
Opera now includes a cool Firebug-inspired developer tool named Opera Developer Console that can be added into the browser with many new features. The developer console includes new tools including DOM inspector, JavaScript inspector, CSS editor and HTTP header inspector.

The Developer Console exposes various aspects of the DOM, the JavaScript environment and the CSS attached to the document. Included tools details:

DOM - The DOM tool lets the user inspect and alter various various aspects of the document.

JS - The JavaScript tool lets the user to inspect and search the JavaScript environment initialized on the web page.

CSS - The CSS tool lets the user inspect and alter any Cascading Style Sheets attached to the page. Further, the tools allow the altered stylesheets to be exported for later use.

HTTP - The HTTP tool allows user to inspect the HTTP headers sent from the server, inspect and edit cookies, and allows logging of XMLHttpRequests sent by the document.

If you using Opera, you can install this tool by from official tools page