RadEditor supports the mobile browsers the rest of the suite supports: http://www.telerik.com/aspnet-ajax/tech-sheets/browser-support.
It relies on the browser's rich text editing engine (although we are trying to decouple it as much as possible and we are constantly improving it) so if some mobile browser has a really important issue, it may also manifest with the RadEditor, but it should generally work fine.
At the moment we are working on creating a lightweight render mode for it, so it works faster and loads less resources. It will also provide elastic design capabilities.
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