Note that the content area of RadEditor is a standard editable iframe element and the rendering in it is managed by the browser and OS.
I have the following suggestions to eliminate this browser behavior:
1) Set the CssFiles property to point to an empty CSS file, e.g.
<telerik:EditorCssFile Value="~/Empty.css" />
Therefore the editor will not load the page styles in its content area and this will eliminate the case when the problem is related to some style.
2) Disable the content filters (ContentFilters="None")
3) Remove the Undo/Redo buttons: Removing Toolbar Buttons
4) Set <compilation debug="false" /> in the web.config file
the Telerik team
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