[KevinH]

Then use an external xhtml validator.

Sigil will load exactly what you give it or will tell you that problems exist and let you decide if you want to auto repair or not. If you decide to not auto repair, you can run sanity check to get a simple list of errors that prevent it from being well formed and fix them yourself. Or make a Checkpoint, then run auto repair and see everything that it fixed using the Checkpoint tools. If you then decide you do not like what was changed you can revert back to the checkpoint made. You can manage your Checkpoint repos to delete them once you have to things the way you want.

As for working with teams, then using the FolderIn and FolderOut plugins to access git repos for the book is what you want.

Thank you for your suggestions but I am done with this subject as you do not appear to be even trying what I suggest.