Workaround: 

ControlsToSkip="TextArea"


Steps to reproduce:

<telerik:RadFormDecorator RenderMode="Lightweight" runat="server" EnableRoundedCorners="true" DecoratedControls="All" />

    <telerik:RadEditor ID="RadEditor1" runat="server"  RenderMode="Lightweight" AutoResizeHeight="true">
        <Content>
            a<br/>a<br/>a<br/>a<br/>
        </Content>
    </telerik:RadEditor>

1. Set this content to a page
2. Toggle Preview mode

Actual behavior: TextArea remain visible on the page as its styles get overriden
Expected: TextArea is hidden