I know morten likes to complain about the CSS in Drupal core. And while he does love arguing, its much easier to change the CSS in core if we have useful metrics to show why the CSS is crappy.
Stubbornella (Nicole Sullivan) has been a long-time advocate of simple CSS to improve front-end performance. Complex selectors take more time for browsers to match against them. Needlessly-complex selectors lead to crazy cold-war-scenario specificity escalation as each rule tries to out-specify the more general rules.
She has also just released an open-source tool called csslint that we can use to test our current CSS rules.
We've declared which rules we choose to follow inline with our CSS standards in
For those wishing to run csslint locally, follow the instructions for the Drupalcore frontend toolkit
User interface changes