Here is the explanation: http://www.w3.org/TR/2010/PR-css3-color-20101028/#transparency http://www.w3.org/TR/2010/PR-css3-color-20101028/#transparen... The CS

Here is the explanation: http://www.w3.org/TR/2010/PR-css3-color-20101028/#transparency http://www.w3.org/TR/2010/PR-css3-color-20101028/#transparen... The CSS3 spec describes that the element with opacity < 1 should be composited off screen. Therefor, an element with opacity < 1 must create a stacking context, because its content layers cannot 'intertwine' with other layers not inside the element. Let's say you'd have a rule that pokes a hole in a layer. If that layer contains several pieces of paper, then the spec would say to take the layer with those pieces of paper aside, poke a hole though all of them, then place the layer back where you found it. This way, other layers on the page are not damaged. /end analogy.