RadEditor is a client-side browser tool, providing the ability to visually edit pure HTML text using the underlying Rich Text Editing engine of the browser. This browser engine supports only standard HTML/XHTML tags and plain text.
Please, note that if you put a server tag in RadEditor the editor will not strip the tag, but will not render it in Design mode.
The ASP and ASP.NET server tags are not HTML tags and the browser is not able to visualize them. The browser cannot visualize XML Tags and when you put <asp:button> in the editor, the server-side framework will not parse the content (as it is part of the editor, not part of the page).
So, instead of sending to the client the HTML tag <input type=button> it will send <asp:button> - which means nothing to the browser.
To understand the situation better, please create a new HTML page with extension .html and add your server tags content you put to the browser and load
it in the browser.
RadEditor is more similar to a desktop HTML editor rather than to an integrated program like MS Word, FrontPage or Visual Studio for web and your scenario is not supported.
the Telerik team