CSS Layout has been transformed over the past years. Smashing Magazine has you covered with all you need to master this new world. If you want to understand the technical details, or learn how to use new CSS creatively, these guides can help.
At some point in our carrers, we've all assiged large values to our component's z-index, and even sprinkled some !important on top, just to be safe. Check this article to get a better idea on how to approach this common problem.