For years I used FireBug to inspect HTML, CSS. But Visual Studio team did great work and now we got even better functionality right inside our favorite IDE. What’s so awesome about it? You can “Inspect” any element on you HTML or CSS and Page Inspector will show you a line in source code that produced that element!
4 minutes introduction video could be found here http://www.asp.net/vnext/overview/videos/visual-studio-2012-page-inspector. (thanks to Scott Hanselman)