The posted link for the online example no longer works. I'm assuming it is now the demo page here: http://demos.telerik.com/aspnet-ajax/tabstrip/examples/appearance/uniquetabappearance/defaultcs.aspx. However I'm not seeing any way to set the CssClass of a tab that has failed validation. Is this functionality that has been removed? I can't find a way to change a tab's appearance when validation has failed.
Also, that demo appears to be broken. I've looked at it in IE, Firefox and Chrome; the CSS seems to be really messed up in every browser and the demo doesn't show off the various states the tabs can be in.