Notice that in the example below the SuperStyle which is based on StartingStyle does not automatically know the TargetType, and is complaining about access to the TextTrimming property. If the TargetType of TextBlock is repeated in the SuperStyle declaration, the error goes away.

By retrieving the SuperStyle from the window’s resources we find that the TargetType is System.Windows.IFrameworkInputElement

and not System.Windows.Controls.TextBlock. So be sure to indicate the TargetType even when using BasedOn to compose a style.