web.config error: Could not find schema information for the element 'case'.

I've added the section below trying to have my menu displayed correctly in the Chrome browser.
However, VS2008 complains:
Message 1 Could not find schema information for the element 'case'.
Message 2 Could not find schema information for the attribute 'match'.
The assemblies listed below are present but clearly one or more is missing. Which please?
/gustav
 
From web.config: 
<add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
< ...