Firebug is a great plugin for developers. It is the best firefox plugin i am using. It is a free, open source for the Firefox web browser. Now you can also use it in chrome. You can on-the-fly you can inspect code, do real-time edition, check sizes, validate. Using Firebug you can see the entire HTML code for that page revealed, so you can go line by line and look at the code on the page, how it functions, and how its working.