Debugging Visual Studio .props files

Question

While trying to debug Visual Studio 2019 configuration problems, I often find myself in a situation where I need to display debug strings in the build output window:

Sometimes I can display a debug string in the build output window by inserting a "one-liner" into a .props file, like this:

...
  <Target Name="ValidateDriverProperties" BeforeTargets="PrepareForBuild">
  <Message  Importance="high" Text="MyDebugMessage: '$(WDKContentRoot)'" />
...

The code fragment above is from: C:\Program Files (x86)\Windows Kits\10\build\WindowsDriver.Common.targets with WDK v10.0.19041.0 installed.

However, when I try to insert my "one-liner" debug message into another .props file, I get an error like this:
error MSB4066: The attribute "Importance" in element <Message> is unrecognized.

...
  <MatchingWdkPresent Condition="Exists('$(WDKContentRoot)\Include\$(TargetPlatformVersion)\km\ntddk.h')">true</MatchingWdkPresent>
  <Message  Importance="high" Text="MyDebugMessage: '$(WDKContentRoot)'" />
...

The code fragment above is from: C:\Program Files (x86)\Windows Kits\10\build\WindowsDriver.Default.props with WDK v10.0.19041.0 installed.

Question: How to reliably display a debug string in the VS build output window from an arbitrary .props file ?

My question is not about the particular problem outlined below, but it might serve as a context:
The $(WDKContentRoot) variable has a trailing backslash and the code: $(WDKContentRoot)\Include\$(TargetPlatformVersion)\km\ntddk.h generates a bad path with double backslash like this: C:\Program Files (x86)\Windows Kits\10\\Include\x64\km\ntddk.h

Answer 1

In case anyone else bumps into this, you get this error if you try to put a Message element in a PropertyGroup... which you can't do. To resolve, move the Message outside the property group (presumably inside some Target block).

The error message is very misleading here...