An element which is very opaque will hide anything behind it. In CSS you can control this. Start by opening relative.html which you created here and saving it as opacity.html. View it and you should have two div elements with the blue one partly covering the green one.
Find the colour definition in the styles for the blue one (second) and add a new line under it:
You should now be able to see the green div "through" the blue one. Play with the opacity value (between 0 and 1).