I use config.ContentsCss to load the live site’s styles into the editor:
config.contentsCss = ['../css/reset.css','../css/screen.css'];
But, many elements have more complex selectors, such as #content h2, and since the editor handles only elements inside <div id="content"> »» WHAT THE EDITOR SEES «« </div>
Example – H2 elements:
Inside screen.css I have this selector:
#content h2 { color: #76828A !important; text-decoration: none !important; }
In the live page it does show the element correctly, but since the editor loads only the text inside #content, it doesn’t being shown correctly.
For reference, I replied to the question when posted in the CKEditor forums: http://cksource.com/forums/viewtopic.php?f=6&t=25962