DIV ONE
div one has moved to the top of the stack because we assigned a z-index of 10. The other divs do not have a z-index assigned so will stack according to the order they appear in our code. #three is second in line so will be in the middle. #two is the last one so will appear on top of #three.
DIV THREE
Normal stacking order is determined by the order in which the elements appear in the code. Div two is the last one listed in the code so it appears at top of the stack. Div three is partially covered by div two and without the padding and opacity declarations I put in there, you wouldn't be able to read what's in this div. Divs two and three are both stacked on top of div one.
DIV TWO
div two is not on top anymore.
Click HERE to see part 1 of this lesson.