This is related to request #42581. The scope for this request is only to warn Artidoc users about the bad heading hierarchy (Artifact sections should not have sections of lower level below them). Release date for rel #41832 is close, so we do not have time to do more. We will prevent actions leading to bad heading hierarchy in request #42581, but for Tuleap 16.7 we "just" warn users.
The mock-up is the following: 
Each "bad child section" will have a red error background and an error message.