There are some neat interactive demos in here from Thiru Manikandan. There are a couple of very tricky things with z-index that never fail to confuse. In addition to things like requiring positioning and source order, the trickiest are the stacking contexts and parent/child relationships. z-index isn’t a flat playing field. Even if you put z-index: 2147483644¹ on an element, it’s possible nothing will happen because that element might be inside a parent element with its own stacking context and a lower z-index than a sibling or some higher-up level DOM element.
Just three shy of the maximum 2147483647. LOLZ. Hat tip to Dan Danney who mentioned seeing that in the wild recently.