<counter>

CSS counters are, in essence, variables maintained by CSS whose values may be incremented by CSS rules to track how many times they're used. This lets you adjust the appearance of content based on its placement in the document. CSS counters are an implementation of Automatic counters and numbering in CSS 2.1.

The value of a counter is manipulated through the use of counter-reset. counter-increment can be displayed on a page using the counter() or counters() function of the content property.

Using counters

To use a CSS counter, it must first be reset to a value (0 by default). To add the value of a counter to an element, use the counter() function. The following CSS adds to the beginning of each h3 element "Section <the value of the counter>:".

body {
  counter-reset: section;                    /* Set the section counter to 0 */
}

h3::before {
  counter-increment: section;                /* Increment the section counter*/
  content: "Section " counter(section) ": "; /* Display the counter */
}

Example:

<h3>Introduction</h3>
<h3>Body</h3>
<h3>Conclusion</h3>

Nesting counters

A CSS counter can be especially useful to make outlined lists because a new instance of a CSS counter is automatically created in child elements. Using the counters() function, a string can be inserted between different levels of nested counters. For example this CSS:

ol {
  counter-reset: section;                /* Creates a new instance of the
                                            section counter with each ol
                                            element */
  list-style-type: none;
}

li::before {
  counter-increment: section;            /* Increments only this instance
                                            of the section counter */
  content: counters(section,".") " ";    /* Adds the value of all instances
                                            of the section counter separated
                                            by a ".". */
                                         /* if you need to support < IE8 then
                                            make sure there is no space after 
                                            the ',' */
}

Combined with the following HTML:

<ol>
  <li>item</li>          <!-- 1     -->
  <li>item               <!-- 2     -->
    <ol>
      <li>item</li>      <!-- 2.1   -->
      <li>item</li>      <!-- 2.2   -->
      <li>item           <!-- 2.3   -->
        <ol>
          <li>item</li>  <!-- 2.3.1 -->
          <li>item</li>  <!-- 2.3.2 -->
        </ol>
        <ol>
          <li>item</li>  <!-- 2.3.1 -->
          <li>item</li>  <!-- 2.3.2 -->
          <li>item</li>  <!-- 2.3.3 -->
        </ol>
      </li>
      <li>item</li>      <!-- 2.4   -->
    </ol>
  </li>
  <li>item</li>          <!-- 3     -->
  <li>item</li>          <!-- 4     -->
</ol>
<ol>
  <li>item</li>          <!-- 1     -->
  <li>item</li>          <!-- 2     -->
</ol>

Yields this result:

Specifications

Specification	Status	Comment
CSS Lists and Counters Module Level 3 The definition of 'CSS Counters' in that specification.	Working Draft	No change
CSS Level 2 (Revision 1) The definition of 'CSS Counters' in that specification.	Recommendation	Initial definition

See also

• counter-reset
• counter-increment

There is an additional example available at http://www.mezzoblue.com/archives/20.../counter_intu/. This blog entry was posted on November 01, 2006, but appears to be accurate.