Stacking context with opacity

CSS box model is a stack of 2-dimensional layers in which the default behaviour is:

  1. Each child element is on top of parent element
  2. For sibling elements, current node appear on top of previous one

Every element in a document is stacked up with the base root element. It is well known that we can alter this stacking bahaviour with change of any of these default properties: z-index: auto, position: auto, transform: none.

Recently I came to know that opacity can also make changes in stacking context!

Since an element with opacity less than 1 is composited from a single offscreen image, content outside of it cannot be layered in z-order between pieces of content inside of it. For the same reason, implementations must create a new stacking context for any element with opacity less than 1. If an element with opacity less than 1 is not positioned, implementations must paint the layer it creates, within its parent stacking context, at the same stacking order that would be used if it were a positioned element with ‘z-index: 0’ and ‘opacity: 1’.

- W3C CSS Color Module Level 3 - Transparency

Here is an example: in the following codepen sample code, Fig. A shows the default stacking with respect to the root of the document. (There are 3 divs - red, green and blue, each surrounded by div containers.)

In Fig. B, red has got z-index: 1 which introduces a new stacking context. All elements are stacked up against root with z-index: auto(or z-index: 0), while red will be raised from the that layer. So it appears on topmost layer.

Question: how can we bring the red layer to the bottom without changing position, z-index and transform?

Answer: use opacity < 1 for parent of red.

The container div of red is set with property opacity: 0.9 in Fig. C, so a new stacking context will be induced in that div. Therefore the z-index: 1 for red will be applied with respect to the parent div, instead of w.r.t root node. Thus it appears below green and blue blocks.

See the Pen BotKr by Praseetha KR (@Praseetha-KR) on CodePen.