Resolving "Validation (): Element ‘xxxx’ is not supported" warning in Visual Studio 2005/2008

Delete the files from “C:\Documents and Settings[Username]\Application Data\Microsoft\VisualStudio\9.0\ReflectedSchemas” folder (or “…\VisualStudio\8.0\…” if running Visual Studio 2005) in Windows XP. In Windows 7 it is under "C:\Users{User Profile}\AppData\Roaming\Microsoft...etc". Remember also the "VisualStudio" part of the path will be different depending on the version installed.

I closed Visual Studio (always a good idea for changes that will affect the IDE), deleted the files then re-opened the project. The warnings were gone.

I found references to this solution at: http://forums.asp.net/t/1205528.aspx http://blogs.msdn.com/mikhailarkhipov/archive/2005/04/21/410557.aspx

A quick Google search found the solution. FYI, the search term I used in Google was “element is not supported”.

I don't know why this happens but I do know there are some flaky domain profile things happening in the network environment.

Another possible solution. When using Master pages, this warning may occur when using ContentPlaceHolders to put elements in document sections where they are not allowed, according to your doctype. E.g., putting a div in the head, under xhtml 1.0 Transitional. Removing the offensive elements (i.e., making your page validate), will make the warning disappear.