CSS Pseudo-elements: The Magic of Creating Virtual Elements

The Essence of Pseudo-elements

Pseudo-elements are special selectors in CSS that allow you to insert "virtual" elements into a document or style specific parts of elements without modifying the HTML structure.

Pseudo-elements use double colon :: syntax (CSS3 spec, but : single colon syntax is still valid for backward compatibility). You can think of pseudo-elements as "ghost elements created by CSS" — they exist in the rendering result but not in the HTML code.

css

/* Modern syntax (recommended) */
selector::pseudo-element {
  property: value;
}

/* Legacy syntax (backward compatible) */
selector:pseudo-element {
  property: value;
}

Difference between Pseudo-elements and Pseudo-classes:

Pseudo-classes (:hover, :focus) select specific states of elements
Pseudo-elements (::before, ::after) create or select specific parts of elements

::before and ::after

::before and ::after are the most commonly used pseudo-elements, inserting generated content before and after element content.

Basic Syntax

css

.element::before {
  content: ""; /* content property is required */
  /* other styles */
}

.element::after {
  content: "";
  /* other styles */
}

Important Rules:

The content property is required, even if it's an empty string
Pseudo-elements are inline elements by default, usually need to set display property
Pseudo-elements are not in the DOM, cannot be directly accessed by JavaScript
Pseudo-elements cannot be applied to replaced elements (like <img>, <input>)

Decorative Icons

css

/* Add icons */
.tip::before {
  content: "💡";
  margin-right: 8px;
}

.warning::before {
  content: "⚠️";
  margin-right: 8px;
}

.success::after {
  content: "✓";
  margin-left: 8px;
  color: #27ae60;
}

/* Using Unicode */
.external-link::after {
  content: "\2197"; /* ↗ */
  font-size: 0.8em;
  margin-left: 4px;
}

/* Using images */
.pdf-link::before {
  content: "";
  display: inline-block;
  width: 16px;
  height: 16px;
  background: url("pdf-icon.svg") no-repeat center;
  background-size: contain;
  margin-right: 6px;
  vertical-align: middle;
}

Quote Marks

css

/* Auto-add quotes */
blockquote::before {
  content: open-quote;
  font-size: 3em;
  color: #3498db;
  line-height: 0;
  margin-right: 10px;
  vertical-align: -0.4em;
}

blockquote::after {
  content: close-quote;
  font-size: 3em;
  color: #3498db;
  line-height: 0;
  margin-left: 10px;
  vertical-align: -0.4em;
}

blockquote {
  quotes: '"' '"' "'" "'"; /* Define quote style */
  font-style: italic;
  padding: 20px;
}

Decorative Shapes

css

/* Ribbon effect */
.ribbon {
  position: relative;
  background-color: #3498db;
  color: white;
  padding: 10px 20px;
  display: inline-block;
}

.ribbon::before,
.ribbon::after {
  content: "";
  position: absolute;
  top: 0;
  width: 0;
  height: 0;
  border-style: solid;
}

.ribbon::before {
  left: -20px;
  border-width: 20px 20px 20px 0;
  border-color: transparent #3498db transparent transparent;
}

.ribbon::after {
  right: -20px;
  border-width: 20px 0 20px 20px;
  border-color: transparent transparent transparent #3498db;
}

/* Tag badge */
.tag {
  position: relative;
  background-color: #e74c3c;
  color: white;
  padding: 5px 10px;
  margin-right: 15px;
}

.tag::after {
  content: "";
  position: absolute;
  right: -15px;
  top: 0;
  width: 0;
  height: 0;
  border-top: 15px solid #e74c3c;
  border-right: 15px solid transparent;
  border-bottom: 15px solid #e74c3c;
}

Counters

css

/* Auto-numbering */
body {
  counter-reset: section;
}

h2::before {
  counter-increment: section;
  content: "Section " counter(section) ": ";
  color: #3498db;
  font-weight: bold;
}

/* Nested counters */
.chapter {
  counter-reset: subsection;
}

.chapter h2::before {
  counter-increment: section;
  content: counter(section) ". ";
}

.chapter h3::before {
  counter-increment: subsection;
  content: counter(section) "." counter(subsection) " ";
}

/* List styling */
ol {
  counter-reset: custom-counter;
  list-style: none;
}

ol li::before {
  counter-increment: custom-counter;
  content: counter(custom-counter) ". ";
  color: #3498db;
  font-weight: bold;
  margin-right: 8px;
}

Displaying Attribute Values

css

/* Display link URL */
a[href]::after {
  content: " (" attr(href) ")";
  font-size: 0.8em;
  color: #777;
}

/* Display abbreviation full name in print styles */
@media print {
  abbr[title]::after {
    content: " (" attr(title) ")";
    font-style: italic;
  }
}

/* Display image alt text */
img::after {
  content: attr(alt);
  display: block;
  text-align: center;
  font-size: 0.9em;
  color: #666;
  margin-top: 5px;
}

::first-letter - First Letter

Selects the first letter of a block-level element, commonly used to create drop cap effects:

css

/* Classic drop cap */
p::first-letter {
  font-size: 3em;
  font-weight: bold;
  float: left;
  line-height: 1;
  margin: 0 10px 0 0;
  color: #e74c3c;
}

/* Decorative first letter */
article p:first-of-type::first-letter {
  font-size: 4em;
  font-family: "Georgia", serif;
  color: #3498db;
  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  float: left;
  padding: 10px;
  margin-right: 10px;
}

/* First letter with border */
.chapter::first-letter {
  font-size: 2.5em;
  font-weight: bold;
  color: white;
  background-color: #2c3e50;
  padding: 10px 15px;
  margin-right: 10px;
  float: left;
  border-radius: 4px;
}

::first-line - First Line

Selects the first line of text in a block-level element:

css

/* Highlight first line */
p::first-line {
  font-weight: bold;
  color: #2c3e50;
  font-size: 1.1em;
}

/* News article style */
article p:first-of-type::first-line {
  text-transform: uppercase;
  letter-spacing: 2px;
  font-weight: 600;
}

/* Combined usage */
.intro::first-line {
  font-variant: small-caps;
  color: #3498db;
}

.intro::first-letter {
  font-size: 2em;
  color: #e74c3c;
}

::selection - Text Selection

Customize the style when users select text:

css

/* Global selection style */
::selection {
  background-color: #3498db;
  color: white;
}

/* Specific element selection style */
code::selection {
  background-color: #2c3e50;
  color: #f39c12;
}

.highlight::selection {
  background-color: #f39c12;
  color: #2c3e50;
}

/* Firefox compatibility */
::-moz-selection {
  background-color: #3498db;
  color: white;
}

::placeholder - Placeholder

Customize the style of form input box placeholders:

css

/* Basic placeholder style */
input::placeholder {
  color: #95a5a6;
  font-style: italic;
  opacity: 1;
}

textarea::placeholder {
  color: #bdc3c7;
  font-size: 0.9em;
}

/* Placeholder change on focus */
input:focus::placeholder {
  color: transparent;
}

/* Different browser prefixes */
input::-webkit-input-placeholder {
  color: #95a5a6;
}

input::-moz-placeholder {
  color: #95a5a6;
  opacity: 1;
}

input:-ms-input-placeholder {
  color: #95a5a6;
}

Practical Applications

Clearfix

css

.clearfix::after {
  content: "";
  display: table;
  clear: both;
}

Loading Animation

css

.loading::after {
  content: "";
  display: inline-block;
  width: 20px;
  height: 20px;
  border: 3px solid #f3f3f3;
  border-top: 3px solid #3498db;
  border-radius: 50%;
  animation: spin 1s linear infinite;
  margin-left: 10px;
  vertical-align: middle;
}

@keyframes spin {
  0% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(360deg);
  }
}

css

.tooltip {
  position: relative;
  cursor: help;
}

.tooltip::before {
  content: attr(data-tooltip);
  position: absolute;
  bottom: 100%;
  left: 50%;
  transform: translateX(-50%);
  background-color: #2c3e50;
  color: white;
  padding: 8px 12px;
  border-radius: 4px;
  white-space: nowrap;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s;
  margin-bottom: 8px;
}

.tooltip::after {
  content: "";
  position: absolute;
  bottom: 100%;
  left: 50%;
  transform: translateX(-50%);
  border: 6px solid transparent;
  border-top-color: #2c3e50;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s;
  margin-bottom: 2px;
}

.tooltip:hover::before,
.tooltip:hover::after {
  opacity: 1;
}

css

.breadcrumb {
  list-style: none;
  display: flex;
  padding: 0;
}

.breadcrumb li {
  display: flex;
  align-items: center;
}

.breadcrumb li:not(:last-child)::after {
  content: "›";
  margin: 0 10px;
  color: #95a5a6;
  font-size: 1.2em;
}

.breadcrumb li:last-child {
  color: #3498db;
  font-weight: bold;
}

Progress Bar

css

.progress {
  width: 100%;
  height: 30px;
  background-color: #ecf0f1;
  border-radius: 15px;
  overflow: hidden;
  position: relative;
}

.progress::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: var(--progress, 0%);
  background: linear-gradient(90deg, #3498db, #2ecc71);
  border-radius: 15px;
  transition: width 0.3s ease;
}

.progress::after {
  content: attr(data-label);
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  color: #2c3e50;
  font-weight: bold;
  z-index: 1;
}

Card Decoration

css

.card {
  position: relative;
  background: white;
  border-radius: 8px;
  padding: 20px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
  overflow: hidden;
}

/* Top decoration bar */
.card::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 4px;
  background: linear-gradient(90deg, #667eea, #764ba2);
}

/* Corner badge */
.card::after {
  content: "NEW";
  position: absolute;
  top: 20px;
  right: -30px;
  background-color: #e74c3c;
  color: white;
  padding: 5px 40px;
  transform: rotate(45deg);
  font-size: 0.8em;
  font-weight: bold;
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
}

Button Hover Effect

css

.button {
  position: relative;
  padding: 12px 24px;
  background-color: #3498db;
  color: white;
  border: none;
  border-radius: 4px;
  overflow: hidden;
  cursor: pointer;
  transition: color 0.3s;
}

.button::before {
  content: "";
  position: absolute;
  top: 0;
  left: -100%;
  width: 100%;
  height: 100%;
  background-color: #2980b9;
  transition: left 0.3s;
  z-index: -1;
}

.button:hover::before {
  left: 0;
}

.button:hover {
  color: white;
}

Divider Lines

css

.section-title {
  text-align: center;
  position: relative;
  margin: 40px 0;
}

.section-title::before,
.section-title::after {
  content: "";
  position: absolute;
  top: 50%;
  width: 40%;
  height: 1px;
  background-color: #ddd;
}

.section-title::before {
  left: 0;
}

.section-title::after {
  right: 0;
}

/* Decorative divider */
.divider {
  text-align: center;
  margin: 30px 0;
  position: relative;
}

.divider::before {
  content: "";
  position: absolute;
  left: 0;
  top: 50%;
  width: 100%;
  height: 1px;
  background: linear-gradient(90deg, transparent, #3498db, transparent);
}

.divider::after {
  content: "✦";
  position: relative;
  display: inline-block;
  padding: 0 20px;
  background-color: white;
  color: #3498db;
  font-size: 20px;
}

Best Practices

Always include the content property: Even if it's an empty string, write content: ""
Pay attention to z-index: Pseudo-elements might cover other content
Performance considerations: Excessive use of pseudo-elements may affect rendering performance
Clear semantics: Use pseudo-elements for decorative content, use HTML for substantive content
Accessibility: Pseudo-element content may not be visible to screen readers
Browser compatibility: Older browsers may require single colon syntax

Summary

Pseudo-elements are one of the most creative features in CSS. They allow us to add rich visual effects and decorative elements without polluting the HTML structure.

Key Points:

::before and ::after: Most commonly used pseudo-elements, insert virtual elements before and after content
- content property is required
- Can be used for icons, quotes, shapes, counters, attribute value display, etc.
- Default as inline elements, often need to set display
::first-letter: Select first letter, implement drop cap and other effects
::first-line: Select first line, highlight first line of text
::selection: Customize text selection style
::placeholder: Customize form placeholder style
Practical Applications: Clearfix, loading animations, tooltips, breadcrumb navigation, progress bars, decorative effects
Best Practices: Use reasonably, pay attention to performance, maintain semantics, consider accessibility

CSS Pseudo-elements: The Magic of Creating Virtual Elements ​

The Essence of Pseudo-elements ​

::before and ::after ​

Basic Syntax ​

Decorative Icons ​

Quote Marks ​

Decorative Shapes ​

Counters ​

Displaying Attribute Values ​

::first-letter - First Letter ​

::first-line - First Line ​

::selection - Text Selection ​

::placeholder - Placeholder ​

Practical Applications ​

Clearfix ​

Loading Animation ​

Tooltip ​

Breadcrumb Navigation ​

Progress Bar ​

Card Decoration ​

Button Hover Effect ​

Divider Lines ​

Best Practices ​

Summary ​