A deep dive into the z-index attribute and its common attribute values: Understanding absolute positioning

Understand the common attribute values ​​​​of absolute positioning: In-depth analysis of the z-index attribute in CSS

In CSS, absolute positioning (absolute positioning) is a commonly used Positioning method, used to precisely control the position of elements on the page. One of the important attribute values, z-index, can help us determine the overlapping order of elements in the vertical direction. In this article, we will analyze the z-index attribute in depth and give specific code examples to help readers better understand and use this attribute.

Before introducing the z-index attribute, let’s first understand the basic concept of absolute positioning. Absolute positioning refers to detaching an element from the document flow and setting its precise position on the page through top, bottom, left, right and other attributes. By default, absolutely positioned elements will overlap other elements. In this case, you need to use the z-index attribute to control their overlapping order.

z-index can be defined as a positive integer, negative integer or auto. A positive integer represents the overlapping order of elements, with larger values ​​overriding smaller values. And negative integers can position elements below other elements. auto means that the browser will determine the overlapping order of elements based on their order in the document flow.

Let us illustrate the use of the z-index attribute through a specific example. Suppose we have a web page layout that includes a page body, a navigation bar, and a popup box. We want the popup to appear at the top of the page and the navigation bar to be above the main body of the page. At this time we can achieve this effect by setting z-index.

First, we need to set the styles of three elements:

<style>
    .main{
        position: absolute;
        top: 100px;
        left: 100px;
        width: 600px;
        height: 400px;
        background-color: #fff;
        z-index: 0;
    }

    .navbar{
        position: absolute;
        top: 50px;
        left: 100px;
        width: 600px;
        height: 50px;
        background-color: #ccc;
        z-index: 1;
    }

    .popup{
        position: absolute;
        top: 200px;
        left: 200px;
        width: 400px;
        height: 200px;
        background-color: #f00;
        z-index: 2;
    }
</style>

<div class="main"></div>
<div class="navbar"></div>
<div class="popup"></div>

In the above code, we define the styles of the three class names of .main, .navbar and .popup respectively. They vary in location and size. Among them, the z-index of .main is set to 0, the z-index of .navbar is set to 1, and the z-index of .popup is set to 2. In this way, .popup will be displayed at the top of the page, and .navbar will cover .main.

Through this example, we can see the role of the z-index attribute. By setting different z-index values, we can flexibly control the overlapping order of elements on the page. This is very useful when designing web page layout, allowing us to reasonably arrange the display order of elements according to our needs.

In addition, some details need to be paid attention to. First of all, only elements with positioning attributes set (such as absolute positioning, relative positioning, etc.) can use the z-index attribute. Second, if multiple elements have the same z-index value, they are stacked in the order they appear in the document flow. Finally, the z-index value of a parent element affects the overlapping order of its children.

To sum up, the z-index attribute is an important attribute used to control the overlapping order of elements. By setting different z-index values, we can flexibly control the display order of elements on the page. When designing web page layout, reasonable use of z-index attributes can help us achieve more complex page effects.

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