What has bigger priority: opacity or z-index in browsers?

I'm coding a "popup window" in JavaScript and I've come across an interesting thing:

The navy square under the popup window is visible even though I would expect it to be hidden. The popup was added after the square, so it should be on the top.

CSS opacity property of the navy square is 0.3. From what I've tried, it seems that every number from the interval (0,1) would yield the same result. If I change it to 1, then it behaves as expected (i.e. the part of the square under the popup is hidden).

I've tried to set the z-index property to 10 for the square and 100 for the popup, but it doesn't change anything.

What am I missing? Why is part of square displayed?

Tested browsers:

• Firefox 3.6.x
• Chrome 4

This is not a bug and is actually how it's supposed to work. It's a bit confusing as the elaborate description of Stacking Contexts doesn't mention anything about it. However, the visual formatting module links to the color module where this particular gotcha can be found (emphasis mine):

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’. If an element with opacity less than 1 is positioned, the ‘z-index’ property applies as described in [CSS21], except that ‘auto’ is treated as ‘0’ since a new stacking context is always created. See section 9.9 and Appendix E of [CSS21] for more information on stacking contexts. The rules in this paragraph do not apply to SVG elements, since SVG has its own rendering model ([SVG11], Chapter 3).

It's not a problem of opacity being more important than z-index, rather than z-index being relative to their stacking context (see z-index in the CSS2 specification).

In other words, z-index are only significant within the context of a positioned ancestor (whether its relative, absolute or fixed). What you need to do to fix your problem is add a position: relative; to the element that contain both your popup and your navy square, and probably add it a z-index: 1; . Seeing your screenshot it will probably be a top element such as a wrapper div.

Workaround for two elements, like divs: add a 0.99 opacity to your top element, and the order of both is reestablished.

opacity: 0.99;

An alternative to using opacity, is to use a transparent colour (with an alpha value)

So, rather than using

{
    background: gray;
    opacity: 0.5;
}

You could try

{
    background: rgba(128,128,128,0.5);
}

It isn't identical, but I was encountering the same issue you were having, and the above fixed it.