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The web developer toolbar has a little-known but powerful tool under the CSS menu – “View Style Information”. (It will be greyed-out unless the DOM inpector is installed.) Click it and then click an element in the page, and it will open a tab or sidebar to display exactly which styles are being applied to the element, using which selectors, and from what stylesheets. It also shows computed styles, I believe. Incredibly useful, especially on a multi-developer production site.