Problem on Visual Studio 2013 "Invalid Markup" for AlternateContent in the namespace "http://schemas.openxmlformats.org/markup-compatibility/2006"

Hi there,

Here are the complain when we try to open the MainWindow:

Error    2    The name "AlternateContent" does not exist in the namespace "http://schemas.openxmlformats.org/markup-compatibility/2006".
Error    2    The name "Choice" does not exist in the namespace "http://schemas.openxmlformats.org/markup-compatibility/2006".

I have read similar problem for visual studion 2012 (no answer), I am surprised it is still there for visual studio 2013.

Anyone got a solution for it?

Gordon Senior Software Developper