Due to this bug users cannot create HTML5 compliant pages with the RadEditor control.

To workaround the problem you can incorporate the JS method provided in this example:

<telerik:RadEditor runat="server" ID="RadEditor1">
    <Content>
        <nav>
            <ul><li>list item1
                </li></ul>
        </nav>
    </Content>
</telerik:RadEditor>

<script type="text/javascript">
    Telerik.Web.UI.Editor.Utils.isBlockElement = function (element) {
        return this.checkForElement(element, /^(?:body|p|div|h[1-6]|form|fieldset|table|thead|tbody|tr|th|td|ul|ol|li|dl|blockquote|address|hr|nav)$/i);
    };
</script>