>> Wondering why the excluded common css are not in gzipped with the rest? Or are they not common after all?
error.css is only sent with errors, is very small, and is the only css sent with an error. Sending all.css with an error would be massive overkill given that it includes all the css several KB worth, instead of just the few bytes with error.css.
the browser specific ones are not included, because, um, they are browser specific, so only sent depending on the browser the user is using. (To be fair we're approaching a time when these can go away as well - but that's some years away still.)