Won't Fix
Last Updated: 08 Apr 2016 12:50 by ADMIN
Rejected with the following description: 

RadEditor performs complex operation in order to manage its content when switching between Design and HTML modes like cloning elements and inserting content in iframe elements. Internet Explorer, however, has some limitations related to that particular operations that prevents the control from preserving the onload attribute (e.g., attributes are not preserved when the iframe is cloned, writing html to an editable iframe doesn't always preserve the event attributes).

In order to handle that scenario you can replace the event attributes with JavaScript events. For example:

                                                                       

<telerik:RadEditor ID="RadEditor1" runat="server" ContentFilters="ConvertToXhtml">
            <Content>
                <html>
                    <head>
                        <script>
	window.onload = function () {
		alert(1);
	}
                        </script>
                    </head>
                    <body >
                        some text
                    </body>
                </html>
            </Content>
        </telerik:RadEditor> 

Note: the ConvertToXhtml filter should be enabled as well.
1 2