The content area by default is an iframe. You should ensure that your code works in a standard editable iframe and it will work in RadEditor too.
The other option is to set the ContentAreaMode to DIV and test withing an editable DIV. Again if your code does not work, test with an editable DIV:
<div contenteditable="true" style="width:300px;height:300px;border: 1px solid red;">sample content</div>
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