Firefox with Firebug is good.

FYI, the developer tools in IE 11 have been completely rewritten and are very, very good


>In IE10 (not sure whether IE9 is the same) there's a 'Trace Styles' tab which shows the HTML hierarchy for the element which makes it easier to determine what is inherited and from where (strike thru indicates a css setting that has been overridden). You can also uncheck the attribute at any level to see how that affects the layout.
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>But in many ways I prefer the Chrome Developer tools for this sort of thing