Neither of you are choir boys, and I’m fed up with your bickering.
If you change the default controls to match the look and feel of something your visitor has never seen before, you run the risk of creating confusion, distrust, or alienation. Even worse, if the controls are poorly made or conceived — and many are — you might make your site less usable. A cardinal sin.
The more I think about it, the real beneficiaries of a uniform UI across browsers aren’t the site visitors, but rather the designers who demand artistic control and the clients who insist the product looks the same everywhere, without understanding that it’s okay (even expected) to have some differences.
Most browsers do not allow images to be cropped using CSS3’s