Look at src/plugins/image/css/image.css.
The majority of this CSS file (starting at the top) is intended for the Image plug-in's UI. That's not actually needed (and thus unnecessary bytes are sent) when you use a custom UI. Once we migrate to aloha/ui-a-la-carte (#747), that will become commonplace.
The minority of this CSS file (starting at the bottom) is intended for "in-editable styling" (What's the right term?). These are not (or at least arguably less) dependent on the UI. This should live in a separate CSS file.
Splitting the css into logical parts would be a good thing.