WebCallOut in html4 working properly, but in HTML5 I have problem with propertie width 100%

Code is very simply

<ISWebDesktop:WebCallOut ID="WebCallOut1" runat="server" CloseBehavior="Latency" DisplayCloseIcon="false" DisplayTitleIcon="False" DisplayTitleText="False" EnableTheming="false" Height="25px" Latency="10" LayoutMode="Simple" PreferredHorizontalPosition="Left" PreferredVerticalPosition="Top" Text="Proszę czekać" Width="100%" RenderingMode="HTML5" EnableViewState="False" >

<ContainerStyle BackColor="#FF9900" Font-Names="Verdana" Font-Size="10pt" ForeColor="White" HorizontalAlign="Center" Width="100%" >
</ContainerStyle>
                            
</ISWebDesktop:WebCallOut>

When I show WebCallOut - control width depends on the length of the text.

Adam