Z-index seems so simple...
From “What No One Told You About Z-Index”:
Z-index seems so simple: elements with a higher z-index are stacked in front of elements with a lower z-index, right? Well, actually, no. This is part of the problem with z-index. It appears so simple, so most developers don’t take the time to read the rules.
I love posts that delve deep into an obscure topic and explain it clearly, and this is a great one.