Yes the demo has been moved to the page that you have found. It does not show how to customize RadTabs on validation failed, but it shows how one can use the CssClass property of the RadTab to customize its appearance. Currently the demo has some broken styles due to recent changes in our demos implementation.
In order to change the CssClass of a RadTab, you could use both the client-side
API, depending on where your validation has been implemented.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.