We work with a lot of multi-site Sitecore installations, and generally feel this is an area Sitecore excels at... however...
The Sitecore content editor interface does have some areas that don't lend it to good multi-site use though, one of which is the rich text editor. Ideally the CSS dropdown in the editor would display CSS classes from a site-specific CSS file. Unfortunately, out of the box Sitecore does not have the facility to do this.
Without the ability to have site-specific CSS classes, the admins for each site in a multi site installation would be forced to share a CSS file and see styles for all of the other sites - not the greatest user experience.
The Solution(s) - old and new
We've solved this for a while by using code similar to what's found on the Sitecore Developer Network. This worked great for a time, but as of Sitecore 6.4 it stopped working.
So what happened? The default.aspx and the code-behind class that are key to the above solution are still present in Sitecore 6.4, but they are not used.
Thankfully, we found a great blog post on the subject by Mark Stiles that pointed us toward the correct solution for Sitecore 6.4+. The revamped rich text editor uses a handful of new classes, and, as always, knowing where to look makes all the difference. A big thanks to Mark, as no one else on the Sitecore Developer Network or elsewhere seems to be talking about this. (Just a note: I would recommend inheriting from Sitecore.Shell.Controls.RichTextEditor.EditorPage and overriding OnLoad, rather than copying it from Reflector or similar as it appears Mark did).
Inheriting from Sitecore.Shell.Controls.RichTextEditor.EditorPage allowed us to support site-specific CSS files in the RTE and keep the level of user-friendliness our clients have come to expect.
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