Over the last couple of days I started getting the following validation warning (green squiggly line at design time) in a Visual Studio 2008 ASP.NET WebForms project:
Validation (): Element ‘xxxx’ is not supported.
Where 'xxxx' was a standard ASP.NET server control (asp:panel, asp:container, asp:textbox)
The application would still compile and run but the warnings were an annoyance.
See my answer for the resolution I found.