Let's say you've just upgraded a site from Drupal 6 to 7 or made some other upgrade, but the new version of the site doesn't look like the old. The HTML output is fundamentally the same, but the new version of the site looks funny. What do you do?
One solution is to load the old and new sites in Firebug, and then compare the computed style tabs, element by element.
If that doesn't work, here's a tip that will let you do a diff of the computed CSS styles for the two sites.