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Overview

Code Syntactic Sugar supports line numbers through CSS counters. Each line is wrapped in a <span> element with the .css_line class, making it easy to add automatic line numbering.

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Basic Implementation

Add the following CSS to your stylesheet to enable line numbers: 
pre code {
  counter-reset: css-line-number;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
}

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How It Works

  1. Reset Counter: counter-reset initializes a counter named css-line-number at the start of each code block
  2. Increment: counter-increment increases the counter for each .css_line element
  3. Display: content: counter(css-line-number) displays the current counter value
  4. Styling: Additional properties control appearance and spacing

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Styling Options

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Basic Line Numbers

pre code {
  counter-reset: css-line-number;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 2em;
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
}

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Advanced Styling

pre code {
  counter-reset: css-line-number;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 3em;
  margin-right: 16px;
  padding-right: 16px;
  text-align: right;
  color: #6e7781;
  border-right: 1px solid #d0d7de;
}

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Dark Mode Support

Adjust line number colors for dark mode: 
/* Light mode */
.css_line::before {
  color: #a4a4a4;
}

/* Dark mode */
@media (prefers-color-scheme: dark) {
  .css_line::before {
    color: #6e7681;
  }
}

/* Or with a dark class */
.dark .css_line::before {
  color: #6e7681;
}

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Line Numbers with Modifiers

When using line modifiers, you can style line numbers differently: 
pre code {
  counter-reset: css-line-number;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 2em;
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
}

/* Highlighted line numbers */
.css_line[data-highlighted-line]::before {
  color: #f59e0b;
  font-weight: bold;
}

/* Added line numbers */
.css_line[data-added-line]::before {
  color: #22c55e;
}

/* Removed line numbers */
.css_line[data-removed-line]::before {
  color: #ef4444;
  text-decoration: line-through;
}

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Preventing Selection

To prevent line numbers from being copied when users select code: 
.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 2em;
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
  user-select: none; /* Prevents selection */
  -webkit-user-select: none;
  -moz-user-select: none;
}
Using user-select: none ensures that when users copy code, line numbers won’t be included in the clipboard.

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Responsive Line Numbers

Hide line numbers on smaller screens: 
pre code {
  counter-reset: css-line-number;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 2em;
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
}

/* Hide line numbers on mobile */
@media (max-width: 640px) {
  .css_line::before {
    display: none;
  }
}

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Custom Starting Number

Start line numbers from a specific value: 
pre code {
  /* Start from line 10 */
  counter-reset: css-line-number 9;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 2em;
  margin-right: 24px;
  text-align: right;
  color: #a4a4a4;
}
Set the counter to one less than your desired starting number, as the counter increments before displaying.

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Complete Example

Here’s a complete example combining line numbers with styling: 
pre {
  background-color: #f6f8fa;
  border-radius: 6px;
  padding: 16px;
  overflow-x: auto;
}

pre code {
  counter-reset: css-line-number;
  font-family: 'Fira Code', monospace;
  font-size: 14px;
  line-height: 1.5;
  display: block;
}

.css_line {
  display: block;
  padding: 0 8px;
}

.css_line::before {
  counter-increment: css-line-number;
  content: counter(css-line-number);
  display: inline-block;
  width: 3em;
  margin-right: 16px;
  padding-right: 16px;
  text-align: right;
  color: #6e7781;
  border-right: 1px solid #d0d7de;
  user-select: none;
}

.css_line:hover {
  background-color: rgba(0, 0, 0, 0.05);
}

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