In-depth analysis of css z-index (with examples)

Students who have done page layout should be very familiar with the z-index attribute. Z-index is a special attribute for web page display. Because the pattern displayed by the monitor is a two-dimensional plane, it has an x-axis and a y-axis to represent the position attribute. In order to represent the three-dimensional concept such as the superposition order of the upper and lower layers of display elements, the z-index attribute is introduced to represent the difference in the z-axis. Indicates the upper and lower three-dimensional relationship of an element in the superposition order.

Elements with a larger z-index value will be superimposed on elements with a smaller z-index value. For positioned objects that do not specify this property, objects with positive z-index values ​​are above them and objects with negative z-index values ​​are below them. (Recommended tutorial: CSS video tutorial)

Simple demonstration

<div style="width:200px;height:200px;background-color:#0e0;"></div>
    <div style="position:relative; top:-50px; width:100px;height:100px;background-color:#00e;"><div>

Two p, the second one moves up 50px, the normal situation should be Like this

The second p covers the first p, and adding the z-index attribute to the second one

<div style="width:200px;height:200px;background-color:#0e0;"></div>
    <div style="position:relative; top:-50px; width:100px;height:100px;background-color:#00e;z-index:-5;"><div>

The result will become Like this, the simplest application of z-index is like this

Only valid for positioned elements

The z-index attribute is applicable to Positioned elements (objects whose position attribute value is relative or absolute or fixed) are used to determine the stacking order of positioned elements in the direction perpendicular to the display screen (called the Z axis). That is to say, if the element is not positioned, it The set z-index will be invalid.

<div style="width:200px;height:200px;background-color:#0e0;z-index:30"></div>
<div style="position:relative; top:-50px; width:100px;height:100px;background-color:#00e;z-index:10;"><div>

Although the z-index of the first p is larger than that of the second p, because the first p is not positioned and its z-index attribute does not work, it will still be covered by the second p. .

Who goes up and who goes down for the same z-index

There are actually two situations for the same z-index

1. If neither element is positioned and the position overlaps, or both elements are positioned and the z-index is the same, the position of the element overlaps, then according to the document flow order, the later one will overwrite the previous one.

<div style="position:relative;width:200px;height:200px;background-color:#0e0;"></div>
<div style="position:relative; top:-50px; width:100px;height:100px;background-color:#00e;"><div>

#2. If neither element has z-index set, the default value is used, one is positioned and the other is not positioned, then the positioned element covers the unpositioned element

<div style="position:relative;top:50px;width:200px;height:200px;background-color:#0e0;"></div>
<div style=" width:100px;height:100px;background-color:#00e;"><div>

Father-child relationship processing

If the z-index of the parent element is valid, then the child element will be consistent with the parent element regardless of whether the z-index is set. Will be above the parent element

<div style="position:relative;width:200px;height:200px;background-color:#0e0;z-index:10;">
        <div style="position:relative;width:100px;height:100px;background-color:#00e;z-index:-5;"><div>
    </div>

Although the child element sets the z-index smaller than the parent element, the child element still appears above the parent element

If the parent The z-index of the element is invalid (not positioned or using the default value), then the z-index setting of the positioned child element takes effect

<div style="position:relative;width:200px;height:200px;background-color:#0e0;">
        <div style="position:relative;width:100px;height:100px;background-color:#00e;z-index:-5;"><div>
</div>

The z-index=-5 of the child element takes effect and is overwritten by the parent element

Child elements between brothers

If the z-index of the sibling element is effective, then the coverage relationship of its child elements is determined by the parent element

<div style="position:relative;width:100px;height:100px;background-color:#0e0;z-index:5;">
        <div style="position:relative;width:50px;height:250px;background-color:#00e;z-index:50;"></div>
    </div>

    <div style="position:relative;width:100px;height:100px;background-color:#0e0;z-index:10;margin-top:4px;">
        <div style="position:relative;width:30px;height:150px;background-color:#e0e;z-index:-10;"></div>
    </div>

Although the z-index of the child element of the first p is relatively high, because its parent element has a lower z-index than the second p, the child element of the first p will be replaced by the second p and its child elements. Overwriting

Application

There is often such a mistake. In the last row of the table, put a td and a p, and click to pop up the subtitles. The menu does some operations such as deletion and modification, but every time the pop-up menu is covered by p in the following lines, like the picture below, the pop-up menu is not at the top of the page.

写个简单的例子看看

<!DOCTYPE html>
<html>
    <head>
        <title>Test</title>
        <style type="text/css" >
            html,body
            {
                height:100%;
                width:100%;
                padding:0;
                margin:0;
            }
            
            .menu
            {
                background-color:#0e0;
                position:relative;
                z-index:10;
            }
            
            .options
            {
                display:none;
                position:absolute;
                top:
                z-index:30;
            }
            
            .options div
            {
                background-color:#00e;
            }
        </style>
    </head>
    <body>
    <table border="1" cellpadding="4px" cellspacing="0">
        <tr>
            <th>Name</th>
            <th>Age</th>
            <th>Options</th>
        </tr>
        <tr>
            <td>Byron</td>
            <td>24</td>
            <td>
                <div class="menu" >
                    <div>Options</div>
                    <div class="options" style="display:block;position:absolute;top:20px;">
                        <div>Opion1</div>
                        <div>Opion2</div>
                        <div>Opion3</div>
                        <div>Opion4</div>
                    </div>
                </div>
            </td>
        </tr>
        <tr>
            <td>Byron</td>
            <td>24</td>
            <td>
                <div class="menu">
                    <div>Options</div>
                    <div class="options" >
                        <div>Opion1</div>
                        <div>Opion2</div>
                        <div>Opion3</div>
                        <div>Opion4</div>
                    </div>
                </div>
            </td>
        </tr>
        <tr>
            <td>Byron</td>
            <td>24</td>
            <td>
                <div class="menu">
                    <div>Options</div>
                    <div class="options" >
                        <div>Opion1</div>
                        <div>Opion2</div>
                        <div>Opion3</div>
                        <div>Opion4</div>
                    </div>
                </div>
            </td>
        </tr>
    </table>
    </body>
<html>

期望样式

实际样式

这时候习惯于增大options 的z-index却发现于事无补，为什么呢？因为每个menu的z-index相同，它们的层叠顺序按文档流顺序，无论子元素z-index调到多大，上面menu的options还是会被下面menu遮盖。这时候我的做法一般是把options放到外面，所有的menu用一个，使menu与options没有父子关系，或者干脆在点击menu的时候把它的z-index调大，这样其子元素就不会被遮盖住了。

最后

本文的例子都是以符合W3C的Chrome浏览器做验证，但在IE6，7 z-index的默认值并不是auto而是0，这样会导致很多奇怪现象，这时候就需要考虑这点了。

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