Note: This is related to working with MasterPages only and NOT nested
master pages.

IDE reports error Unrecognized tag prefix or device filter 'asp' when
MasterPageFile attribute is moved from the @Page into Web.config.

In some projects, developers may want to control the Theme and the
MasterPageFile from the Web.config.  That works for Themes, but not for
MasterPageFile, because removing this Page attribute renders the page
unusable in the IDE.  The wierd thing is that the application continues
to work at runtime.

If this is not intended to work as the Themes do, why is it possible to
place a masterPageFile directive in the Web.config>system.web>pages
space?  It will never work, since every page has to have
MasterPageFile...

Steps:
Create a web project.
Create a masterpage.
Create a page that uses the masterpage and add some webcontrols to it.
Remove the masterpagefile directive from the @Page.
Place the masterpagefile directive in web.config.
Build the solution.

Results:
All webcontrols will report error: Unrecognized tag prefix or device
filter 'asp'

Expected results:
Page should build just fine just as it does when a Theme is moved from
@Page into the Web.Config