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Reveal the operating principle and unique characteristics of absolute positioning

王林Original: 2024-01-23 08:15:06503browse

Absolute positioning is a positioning method in CSS that allows an element to be positioned relative to the nearest positioned ancestor element it contains. If there is no positioned ancestor element, The element will then be positioned relative to its original containing block. The working principle and unique characteristics of absolute positioning make it one of the important technologies in web development.

The working principle of absolute positioning can be simply summarized as follows: an element is positioned relative to its nearest positioned ancestor element. This means that we can control the precise position of an element on the page by setting its position attributes (top, bottom, left, right). In contrast, relative positioning positions an element relative to its position in the normal document flow and still preserves the element's space in the document flow when positioned.

Absolute positioning has the following unique characteristics:

The element is detached from the document flow: Through absolute positioning, the element can be detached from the document flow and no longer occupies the original spatial position, which makes the page The layout is more flexible and diverse. However, it should be noted that elements that break away from the document flow will have an impact on the layout of other elements, which may cause elements to overlap or become misaligned, so you need to carefully adjust the position of elements when using absolute positioning.
Precise positioning: By specifying the position attribute of the element, we can accurately position the element anywhere on the page. In this way, we can achieve more detailed page layout and design effects. For example, we can place a picture in the upper right corner of the page, or position a menu in the lower left corner of the page, etc.

The following uses a specific code example to illustrate the working principle and characteristics of absolute positioning. Consider a simple web page layout, which contains a container element with a block of body and an absolutely positioned box element:

<!DOCTYPE html>
<html>
<head>
  <style>
    body {
      position: relative;
    }
    .container {
      width: 500px;
      height: 300px;
      border: 1px solid black;
    }
    .box {
      position: absolute;
      top: 50px;
      left: 50px;
      width: 200px;
      height: 150px;
      background-color: red;
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="box"></div>
  </div>
</body>
</html>

In the above code, the container element .container sets the width, height and border style, Its containing block is the body element. The .box element uses absolute positioning. By setting the top and left attributes, it is positioned at (50px, 50px) of the container element. In this way, the .box element is removed from the document flow and positioned at the specified position of the container element.

By observing the running results, we can clearly see that the .box element is positioned relative to the .body element, not relative to the container element itself. This is exactly how absolute positioning works.

It is worth noting that when we set the position of the .box element to (0, 0), it will cover the border of the container element because the default stacking order of its sibling elements is from front to back . If we want to avoid this situation, we can set the stacking order of elements through the z-index attribute.

To sum up, absolute positioning is a very powerful and flexible CSS technology, which allows us to control the layout and position of elements more precisely. However, when using absolute positioning you need to be aware of overlapping and stacking issues between elements and the possible impact on document flow. Only by using absolute positioning rationally can we bring better results to web page layout and design.

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