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.