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Firebug is a browser extension that allows you to edit live on the site and also to check for errors. It is an indispensable free tool that everyone must learn to employ.
I like this tool so much that it is virtually all I use to make and edit themes. Once you get the hang of it you will ask yourself why you did not use it sooner.
Here are a few videos that will help you learn how to make the best of this excellent tool.