Every time I open up a Windows Phone Project in Visual Studio 2013 I got a lots of errors in the XAML markup. The designer shows "Invalid markup" and the error list is long. I found a workaround for this:
- Open Configuration Manager for your Visual Studio solution
- Change platform for the Windows Phone Project to x86
- Build the Project and the errors is gone
- Open Configuration Manager again and change back to "Any CPU"
After follow above steps it will work until you start Visual Studio again.