Explore the basic concepts and usage scenarios of CSS pseudo-classes and pseudo-elements

Understand the basic concepts and application scenarios of CSS pseudo-classes and pseudo-elements

CSS (Cascading Style Sheets) is a markup language used to describe web page styles. You can control the appearance and layout of elements in a web page. In CSS, pseudo-classes and pseudo-elements are very useful features that can further expand the application scope and flexibility of CSS.

1. Pseudo-classes

Pseudo-classes are keywords used to select specific status elements. Common pseudo-classes include: hover, active, focus, etc. The following are examples of the use of some common pseudo-classes:

1. :hover pseudo-class

:hover pseudo-class is used to select the state of the mouse hovering over the element.

a:hover {
  color: red;
}

2. :active pseudo-class

:active pseudo-class is used to select the state of the element when it is clicked.

button:active {
  background-color: blue;
}

3. :focus pseudo-class

:focus pseudo-class is used to select the focused input element.

input:focus {
  border: 2px solid red;
}

2. Pseudo elements

Pseudo elements are keywords used to insert specific content before or after the content of the element. Common pseudo-elements include: before and after. The following are some common examples of pseudo-element usage:

1. ::before pseudo-element

::before pseudo-element is used to insert specified content before the content of the element.

p::before {
  content: "before";
  color: red;
}

2. ::after pseudo-element

::after pseudo-element is used to insert specified content after the content of the element.

p::after {
  content: "after";
  color: blue;
}

3. Application Scenarios

Pseudo classes and pseudo elements have many practical application scenarios in web page style design. The following takes a common requirement as an example to illustrate the use of pseudo classes and pseudo elements.

Suppose we have a navigation bar on a web page with several navigation links. We want the navigation link's color to change when the mouse is hovered over it, and to add a vertical line separator between the navigation links.

HTML code is as follows:

<div class="nav">
  <a href="#">Home</a>
  <a href="#">About</a>
  <a href="#">Contact</a>
</div>

CSS code is as follows:

.nav a {
  color: black;
  text-decoration: none;
  padding: 10px;
}

.nav a:hover {
  color: red;
}

.nav a:not(:last-child)::after {
  content: "|";
  padding-left: 10px;
  padding-right: 10px;
  color: gray;
}

In the above code, we first set the default color and style for the navigation link. Then using the :hover pseudo-class selector, when the mouse is hovering over the navigation link, the color of the link changes to red.

Next, we use the :not pseudo-class selector to select other links except the last navigation link. Then use the ::after pseudo-element to add a pipe separator after each link and style the separator.

Through the above code, we can achieve the effect of the navigation link changing color when hovering, and adding vertical line separators between links.

Summary:

Pseudo classes and pseudo elements play a very important role in CSS and can help us achieve more flexible and complex style effects. By properly applying pseudo-classes and pseudo-elements, we can better control web page styles and improve user experience. The above example is just one of them. In fact, there are more application scenarios for pseudo-classes and pseudo-elements, which require our continuous exploration and practice in actual development.

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