Unrecognized tag prefix or device filter 'asp'

The solution to this error can be found here.
The short version – if you’re using (nested) master pages and you’re seeing all of your tags underlined and complaining with “Unrecognized tag prefix or device filter ‘asp’.”, the solution is to keep your (nested) master page open in Visual Studio. That’s right – it sounds ugly – but that’s the fix.
So, if you have a Page that references a master page called ChildMaster.master which references ParentMaster.master, you should open ChildMaster.master (even if you don’t plan to make any changes to it) and leave it open in the VS IDE while you work on pages that refer to it.
Hopefully this will be fixed in the upcoming update.