Browser Internet Explorer Netscape Opera Safari Firefox
Version 5.5 6.0 7.0 8.0 8.0 9.0 7.0 8.0 9.2 9.5 1.3 2.0 3.1 1.5 2.0 3.0
Supported Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Short information
Default value auto
Inherited No
Applied To all docked elements
HTML analog No
Reference to specification http://www.w3.org/TR/CSS21/box.html#border-color-properties
Any positioned elements on a web page can be superposed each other in a certain order. Thus they are simulating the third measurement that is perpendicular to the screen. Each element can be both below and above placing other objects on a z-axis. The attribute z-index operates here.


z-index: number | auto
It is possible to use positive or negative integer and also a zero as arguments. The more value the above is an element in comparison with those elements in which a value is less. There is the element in HTML code at equal value z-index which is described below. Though the specification also allows to use negative values z-index but such elements are not displayed in the following browsers: Netscape, Mozilla and Firefox.
There is В«AutoВ» that is applied apart from numerical values. The elements order is under construction automatically proceeding from their position in code HTML and a membership to the parent. It is because of affiliated elements have the same number that their parental element.


   #layer1, #layer2, #layer3, #layer4 {
    position: relative; /* Relative positioning */
   #layer1, #layer3 {
    font-size: 50px; /* Font size in pixels */
    color: #000080; /* Blue text color */
   #layer2, #layer4 {
    top: -55px; /* Top text shift */
    left: 5px; /* Shift text to the right */
    color: #ffa500; /* Orange text color */
    font-size:70px;  /* Font size in pixels */
   #layer1 { z-index: 2; }
   #layer2 { z-index: 1; }
   #layer3 { z-index: 3; }
   #layer4 { z-index: 4; }
Layer 1 above
<div id="layer1">Layer 1</div>
<div id="layer2">Layer 2</div>
Layer 4 above
<div id="layer3">Layer 3</div>
<div id="layer4">Layer 4</div>
The result of this example is shown on the figure 1

Object model
The list created by the tag <SELECT> in the browser Internet Explorer 6 is always imaged above other elements independent from the value "z-index".
Only beginning from the version 5.5 Internet Explorer allows to use the attribute "z-index" to the frames (tag <FRAME>) and float frames (<IFRAME>).
Firefox 3 understands the negative value of "z-index" reasonably.
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