Ok... I'm not sure to have a css in conflict... So, there only is the Asp.net page:
<telerik:radeditor runat="server" ID="DetailsPratiques"
ToolsFile="~/App_Data/RadEditLite.xml"
SkinID="DefaultSetOfTools"
width="95%"
Height="300px">
<CssFiles>
<telerik:EditorCssFile Value="~/view/assets/EditorContentArea.css" />
</CssFiles>
<Tools>
<telerik:EditorToolGroup>
<telerik:EditorDropDown Name="TranslateTool" Text="<img src='/view/assets/flags/fr.png' alt=''/> Translation Tool"
Width="130px" ItemsPerRow="1" PopUpWidth="120px" PopUpHeight="260px">
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/fr.png' alt=''/> French" Value="fr" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/en.png' alt=''/> English" Value="en" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/es.png' alt=''/> Spanish" Value="es" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/de.png' alt=''/> German" Value="de" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/it.png' alt=''/> Italian" Value="it" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/ru.png' alt=''/> Russian" Value="ru" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/pt.png' alt=''/> Portuguese" Value="pt" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/he.png' alt=''/> Hebrew" Value="he" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/nl.png' alt=''/> Dutch" Value="nl" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/ja.png' alt=''/> Japanese" Value="ja" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/bg.png' alt=''/> Bulgarian" Value="bg" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/ro.png' alt=''/> Romanian" Value="ro" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/uk.png' alt=''/> Ukrainian" Value="uk" />
<telerik:EditorDropDownItem Name="<img src='/view/assets/flags/sv.png' alt=''/> Swedish" Value="sv" />
</telerik:EditorDropDown>
<telerik:EditorTool Name="InsertVideo" Text="Multimedia Content" />
</telerik:EditorToolGroup>
</Tools>
</telerik:radeditor>
Maybe it's important, this is in an ajax accordion :
<ajaxToolkit:Accordion
ID="accordion1" runat="server"
FadeTransitions="false" FramesPerSecond="100" TransitionDuration="50"
CssClass="jqtheme" HeaderCssClass="header" ContentCssClass=""
RequireOpenedPane="True" AutoSize="None">
<Panes>
<ajaxToolkit:AccordionPane ID="panier" runat="server">
<Header>
<div>
<li>Informations principales</li>
</div>
</Header>
<Content>
whitch is closed.
(thank you for your help and sorry for my english and annoying you with that)