I know about Workspaces recently introduced in DevTools but that is not that i need. For example: page uses jquery that is loaded from CDN, i modify jquery library code, pre
Just a tip, under Firefox i use Greasemonkey. I dont know if you know this tool, but it allows to run javascript scripts over a webpage for a website. Then, with an adapted javascript you can change css dynamically.
A little get started for greasemonkey: http://www.webmonkey.com/2010/02/get_started_with_greasemonkey/
For chrome it is tampermonkey : http://tampermonkey.net/faq.php