Layering is a very standard and extensively documented HTML feature (z-index). It is just like width, font or size. I do not feel there is a compelling reason to document it (for the record I am not in charge, before anyone suggests I evading my duties).
CSS operates in three dimensions - height, width and depth. For that purpose, you can assign each element a number (z-index). The system is that an element with a higher number overlaps an element with a lower number.
The HTML editors set something like that:
#topmost {
position: absolute;
left: 10px;
top: 10px;
z-index: 16;