Analyze the reasons why fixed positioning is limited in HTML

Analysis of the reasons for limited fixed positioning in HTML requires specific code examples

In CSS, fixed positioning (fixed positioning) is a very useful attribute. It allows us to fix the element at a specific position in the browser window so that it does not change as the page scrolls. However, there are some limitations to fixed positioning in HTML. Below we will analyze the reasons and illustrate them with specific code examples.

1. Parent element is not HTML

Fixed positioning elements are always positioned relative to their nearest ancestor element with positioning. If the ancestor element is not an HTML element, but a parent element with fixed positioning, then the fixed positioning will be restricted.

For example:

<div class="parent">
  <div class="child">
    <p>这是一个固定定位的元素</p>
  </div>
</div>

CSS style:

.parent {
  position: relative;
}

.child {
  position: fixed;
  top: 50px;
  left: 50px;
}

In this example, although the .child element has fixed positioning, its ancestor element .parent has relative positioning , so the position of the .child element is positioned relative to the .parent element, not relative to the entire HTML document.

2. The parent element is not tall enough to contain the fixed-positioned element

When an element has fixed positioning, it breaks out of the document flow and takes up no space in the document. If the parent element is not tall enough to contain the fixedly positioned child element, the child element will exceed the scope of the parent element.

For example:

<div class="parent">
  <div class="child">
    <p>这是一个固定定位的元素</p>
  </div>
</div>

CSS style:

.parent {
  height: 100px;
  background-color: #f1f1f1;
}

.child {
  position: fixed;
  top: 50px;
  left: 50px;
}

In this example, the height of the parent element .parent is 100px, but the positioning of the child element .child is relative It is performed in the browser window, so the position of the .child element will exceed the scope of .parent, causing part of the content to be blocked.

3. The containing block of the child element is incorrect

The containing block of a fixed-positioned element is the reference element that determines its positioning. It is determined by the nearest positioned element. Determined by the ancestor elements. If the containing block is set up incorrectly, fixedly positioned elements will exhibit incorrect positioning.

For example:

<div class="parent">
  <div class="child">
    <p>这是一个固定定位的元素</p>
  </div>
</div>

CSS style:

.parent {
  position: relative;
  width: 300px;
  height: 300px;
  background-color: #f1f1f1;
  overflow: hidden;
}

.child {
  position: fixed;
  top: 50px;
  left: 50px;
}

In this example, the overflow of the parent element .parent is set to hidden, resulting in the positioning reference of the child element .child The element has changed so that its position is no longer relative to the parent element, but is positioned relative to the document.

In actual development, in order to avoid these limitations, we should set the positioning, width and height of the parent element reasonably, and ensure that the containing block of the fixed positioning element is set correctly. Therefore, in the layout and design stage, it is very important to correctly understand and use the principle of fixed positioning.

Summary

This article analyzes the reasons why fixed positioning is limited in HTML and explains it with specific code examples. The main reasons include that the parent element is not HTML, the height of the parent element is not enough to contain the fixed-positioned child element, and the containing block of the child element is set incorrectly. By understanding these limitations, we can make better use of fixed positioning and avoid unnecessary problems.

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