RadEditor uses the browser's rich-text editing (RTE) engine, which works with HTML objects and when some tag is not well formed, i.e. the tag is not properly closed (there is only opening tag for example <DIV>) or the opening tag is missing (there is only a closing tag </DIV>), then the browser automatically correct this by closing the tag or adding the opener tag (<DIV></DIV>).
The RTE engine is used by the editable DIV and IFRAME elements and you can achieve your scenario by pasting the content directly in an editable IFRAME or DIV elements, the browser will correct the non well formed content and you will be able to obtain it cleaned from the IFRAME and set it to your Preview mode.
You can obtain the content of the editable iframe with the following code:
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