The disabled tabs don't look like disabled in Classic render mode. 

This issue is not observable with the 2016 release as shown in the attached screenshot.

Workarounds:
1) You can change the color of the disabled items by applying the following class to the page:

        .rtsLink.rtsDisabled {
            color: #a5a5a5 !important;
        }

2) Another option is to switch the rendering to Lightweight -> RenderMode="Lightweight".

Please see the ticket here http://www.telerik.com/account/support-tickets/view-ticket?threadid=1010098.

I still need to support IE8 browsers.  The removal of the frameBorder attribute from the iframe rendering causes layout issues in IE8.  Adding a property to the RadPageView project to re-enable this behavior would fix this issue.  It could be false by default in order to pass the cited validation tool.  The developer would have to enable the property in order for the frameBorder attribute to be rendered as frameBorder="0".

Please see the following Forum topic and attached images:

http://www.telerik.com/forums/multi-row-tabs#i-uWt1OdnEKceUHt32s43Q

We would consider this to be a bug, and the response we received back from Ivan Danchev was less than satisfactory given that this topic is now over 5 years old.

Please consider adding this to your roadmap and addressing this behaviour sometime in the near future.

Observe the below video, demonstrating the issue :

http://screencast.com/t/pC1V29UwYQ83 

Adding another, not visible textarea to absorb the focus can be used as a workaround:

<telerik:RadTabStrip ID="rtsRFT" runat="server" Skin="Office2007" MultiPageID="rmpRTF"
    SelectedIndex="0" ShowBaseLine="True" OnClientTabSelected="OnClientTabSelected">
    <Tabs>
        <telerik:RadTab Text="Description" TabIndex="61" ToolTip="Click to view description"
            SelectedCssClass="SelectedTab" PageViewID="pDesription">
        </telerik:RadTab>
        <telerik:RadTab Text="Notes" TabIndex="63" ToolTip="Click to view notes" PageViewID="pNotes">
        </telerik:RadTab>
    </Tabs>
</telerik:RadTabStrip>
<div class="welcomeTab" style="height: 100%">
    <telerik:RadMultiPage ID="rmpRTF" runat="server" SelectedIndex="0">
        <telerik:RadPageView ID="pDesription" runat="server">
            <div>
                <telerik:RadEditor ID="txtDescription" runat="server" Width="100%" Height="250px">
                </telerik:RadEditor>
            </div>
        </telerik:RadPageView>
        <telerik:RadPageView ID="pNotes" runat="server" Height="100%">
            <div>
                <telerik:RadEditor ID="txtNote" runat="server" Width="100%" Height="250px">
                </telerik:RadEditor>
            </div>
        </telerik:RadPageView>
    </telerik:RadMultiPage>

</div>

<script type="text/javascript">
    function OnClientTabSelected(sender, args) {
        $telerik.$("#inboxId").focus();
    }
</script>

I work in an environment of Visual Studio 2013 
C # aspx 
ie10 
See the attached project.
After pressing the Send key twice, I get the message below.
The control ID is generated automatically by Rad.

Thanks in advance 

KAMARA YAIR

Email

KAMARA.Y@LAVIETIMETECH.COM

Exception details:

Unhandled exception at line 15, column 9936 in http://localhost:53353/Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=RadScriptManager1_TSM&compress=1&_TSM_CombinedScripts_=;;System.Web.Extensions,+Version=3.5.0.0,+Culture=neutral,+PublicKeyToken=31bf3856ad364e35:he-IL:eb198dbd-2212-44f6-bb15-882bde414f00:ea597d4b:b25378d2;Telerik.Web.UI,+Version=2014.1.403.35,+Culture=neutral,+PublicKeyToken=121fae78165ba3d4:he-IL:64f74e2d-5497-4ab7-8321-89bccc22f4bb:16e4e7cd:86526ba7:874f8ea2:f7645509:24ee1bba:e330518b:1e771326:88144a7a:8e6f0d33:6a6d718d:ed16cbdc

0x800a139e - JavaScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Multiple controls with the same ID 'ctl02' were found. FindControl requires that controls have unique IDs.

1)If already one tab opened means-->just i want to select tab(without postback)

for that me use set_selected(true) and select().But its also fires OnClientTabSelected Client Side API function.

So how can i achieve this?

 OnClientTabSelecting function available to get focus of user select a tab.Becoz i want two functionality

1)I want  postback for some specific of the tabs when user clicks already opened tabs

2)I want to select a Tab(without postback) when user clicks from Leftmenu(RadPanelBar)

Pls explain with sample code how to achieve both Its very urgent

3) In my left side user control  RadPanelItem me used.Based on this only tabs are opened based

Functionality me need -->whenever user select a already opened tabs its corresponding menu also should select in RadPanelbar(PanelItem)

Me tried this logic
============
function ClientTabsSelected(sender, args) {
var tab = args.get_tab();
GetItemText(tab.get_text());
}

    function GetItemText(currenttabtext) { //This function me put left side user control.

          var panelBar = $find("<%=RadPanelBar1.ClientID%>");

           var currentitem = panelBar.findItemByText(currenttabtext);

           if (currentitem) {
               currentitem.set_selected(true);
               currentitem.expand();
          }

      }
================================

Its works only when panelitem expand.If already selected item in Panelbar collapsed means.Its select properly (rpSelected CSS Class assigned  me find using inspect element)

But its not expand..Please help me solve this also

Please response soon.Its very urgent

i have create tab by using tabstrip and assing pageview to that tab but on delete of tab the page view is not deleting .for deleteing  i have use close imagebutton,tab will delete but pageview is not delete.pls suggest me how to delete tab alone tab alone with pageview.

This markup:
<Telerik:RadTabStrip ID="detailsStrip" runat="server" MultiPageID="RadMultiPage1"
                            ClickSelectedTab="true" OnTabClick="detailsStrinp_TabClick" SelectedIndex="0"
                            Align="Justify" Orientation="VerticalLeft" EnableEmbeddedSkins="false" AutoPostBack="true"
                            Skin="WFS" ScrollChildren="true" OnClientTabSelected="onTabSelected">
                            <Tabs>
                                <Telerik:RadTab Text="General"  PageViewID="GeneralView">
                                </Telerik:RadTab>
                                <Telerik:RadTab Text="History" PageViewID="HistoryView">
                                </Telerik:RadTab>
                                <Telerik:RadTab Text="Dokumenty" PageViewID="DocsView">
                                </Telerik:RadTab>
                                <Telerik:RadTab Text="AddInfo" PageViewID="AddInfoView">
                                </Telerik:RadTab>
                                <Telerik:RadTab Text="Gallery" PageViewID="GalleryView">
                                </Telerik:RadTab>
                                <Telerik:RadTab Text="Process" PageViewID="ProcessView" Visible="false" />
                            </Tabs>
                            <TabTemplate>
                                 <div class="nav-item">
                                    <div class="nav-item-img"></div>
                                    <div class="nav-item-header"><%# DataBinder.Eval(Container,"Text") %></div>
                                </div>
                            </TabTemplate>
                        </Telerik:RadTabStrip>

Generates js issue:
if(D.TabStripOrientation.isVertical(H)){G="offsetHeight";
F=function(J,K){J.firstChild.firstChild.firstChild.style.height=K+"px";
(null reference)
Version: 2012.3.1016.35

Currently the RadTabStrip + Google Maps integration suffers a limitation e.g. if the map is within a hidden tab, there are displacements.

While it is currently possible to target the first or last item in a RadTabStrip via their CSS classes .rtsFirst and .rtsLast, it is not possible to target individual items, e.g. to add a custom icon or display different background images in tabs.

Assigning icons via the ImageURL property is often not feasible, e.g. when using the RadTabStrip-based navigation controls in Sitefinity.

Numbering the list items, e.g. .rtsLI1, .rtsLI2, .rtsLI3, etc., would be trivial to implement and provide a lot of flexibility through individual CSS targeting.

This also applies to the RadPanelBar and RadMenu controls.