CSS: z-index Not Working?

A better understanding of the relation between z-index, stacking order, positioned, and non-positioned elements.

Many people are struggling with the z-index. Sometimes, z-index seems to not work. Even setting the z-index to 9999999999 cannot help in putting the element closer to the screen. In this article, we are going to talk about the z-index, stacking order, positioned, and non-positioned elements.