This online demo also uses an external CSS file for decoration (http://demos.telerik.com/aspnet-ajax/editor/examples/settingcontentareadefaults/defaultcs.aspx
). Running it under Firefox and Chrome I see that everything works fine.
Therefore, I am unable to properly define what might have went wrong on your end. And why the CSS file is not loaded.
Note that external CSS decoration in RadEditor is entirely based on native browser technology. Therefore, I suggest you first to investigate whether the CSS file can be loaded properly on the page.
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