Z-index
A bit of a strange one, but we needed something for Z. This controls the order of items in HTML in terms of how they stack ontop of each other. The higher the z-index, the nearer the top of the stack they are.
So, for example, an item with a z-index of 10 sits above (on top of) an item with a z-index of 5.
Without any z-index CSS, elements appear in the order they’re included in the HTML.