Earlier thread on this problem is locked but I am still looking for a solution. Try the simplest possible code with the Tabs control and you get a blank page in IE8:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="TabsSample.Default" %><!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"><head runat="server"><title></title></head><body><form id="form1" runat="server"><ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" /><div><ajaxToolkit:TabContainer runat="server"
Height="150px"
Width="400px"
ActiveTabIndex="0"
OnDemand="true"
AutoPostBack="false"
TabStripPlacement="Top"
CssClass="ajax__tab_xp"
ScrollBars="None"
UseVerticalStripPlacement="false"><ajaxToolkit:TabPanel runat="server"
HeaderText="Labs"
Enabled="true"
ScrollBars="Auto"
OnDemandMode="Once"><ContentTemplate>
Lab test results go here</ContentTemplate></ajaxToolkit:TabPanel><ajaxToolkit:TabPanel runat="server"
HeaderText="Radiology"
Enabled="true"
ScrollBars="Auto"
OnDemandMode="Once"><ContentTemplate>
Radiology results go here</ContentTemplate></ajaxToolkit:TabPanel><ajaxToolkit:TabPanel runat="server"
HeaderText="Medical"
Enabled="true"
ScrollBars="Auto"
OnDemandMode="Once"><ContentTemplate>
Medical test results go here</ContentTemplate></ajaxToolkit:TabPanel></ajaxToolkit:TabContainer></div></form></body></html>