This is a great question and something I've pondered. I recently learned that mixing tabs and space in certain Python environments, such Google Cloud Functions, simply isn't allowed -- uploading such code will cause an error.

Unfortunately, that might be the cleanest solution for Margin, too: to simply disallow the intermingling of tabs and spaces as hierarchical tokens within a single parent.

In that scenario, you could use tabs in one part of the document and spaces in the other -- as long as all the direct children of one parent item follow the same rules.