Z-Index Relative or Absolute?

Question

I\'m trying to find an answer to the following question:

Is an element\'s z-index style its absolute stack order, or its stack order relative to its parent?

Answer 1

z-index is relative. See this detailed answer, which I wrote for a similar question.

If none of the other elements have a defined z-index, using z-index: 1 will be fine.

Model: How is the z-index determined?

                                         z-index
                                Auto 1
    
                            Auto 1.1
       
          Manual 1
       
                   Auto 1.1.2
    
                                
    
                      Auto 1.2

                          Auto 2

First, the direct child nodes of the body are walked through. Two elements are encountered: #A and #F. These are assigned a z-index of 1 and 2. This step is repeated for each (child) element in the document.

Then, the manually set z-index properties are checked. If two z-index values equal, their position in the document tree are compared.

Your case:

          Z-index 1
    
 Z-index 3

    Z-index 2

You'd expect #Y to overlap #Z, because a z-index of 3 is clearly higher than 2. Well, you're wrong: #Y is a child of #X, with a z-index of 1. Two is higher than one, and thus, #Z will be shown over #X (and #Y).

Here is a plunker to visualize this a little better, or try the snippet below ,

.redbox,
.greenbox,
.bluebox {
  height: 100px;
  width: 100px;
  position: relative;
  color: #fff;
  padding: 3px;
}

.redbox {
  background: red;
  z-index: 1;
}

.greenbox {
  background: green;
  top: 40px;
  left: 40px;
  z-index: 3;
}

.bluebox {
  background: blue;
  top: -20px;
  left: 20px;
  z-index: 2;
}

z: 1
  
 z: 3

z: 2