transform-style does not describe keywords and the behavior on the keywords. Shouldn't it be described what flatten behavior actually means?
As an example. I couldn't find text that says that flatten forces the engine to paint in z-index order. In fact, no implementation does it completely.
Safari flattens, planes are still in a virtual 3D context order (no perspective). Firefox flattens in z-index order, but does something different if planes intersect.