CSS Counters

Counters.  They were a staple of the Geocities / early web scene that many of us "older" developers grew up with;  a feature then, the butt of web jokes now.  CSS has implemented its own type of counter, one more sane and straight-forward than the ole "hit counter."  CSS counters allow for simple CSS-based incrementing and display of a number for generated content.  Let's have a quick look at how CSS counters work!

Initializing the CSS Counter

For the sake of semantics, let's displays a series of slides using <OL> and <LI> elements for our counter example.  The first step is resetting the counter, providing the specific counter a name initializing it to 0:

ol.slides {
	counter-reset: slideNum;
}

The counter name slideNum will be used throughout the example.

Incrementing the CSS Counter

To increment the counter, use the counter-increment property and the counter name created in the first step:

ol.slides > li {
	counter-increment: slideNum;
}

Each counter-increment call continues to increment the counter.  Be aware that you must use > in your selector if you have nested lists so that your list numbering doesn't get incremented at the wrong points.

Using the Counter Value

Counters aren't much good if you can't display them (just like in 1995!), so the counter() command will output the counter value when used with the content property:

ol.slides li:after {
	content: "[" counter(slideNum) "]";
}

Oddly enough, counters can also be nested by using a second argument to counter, which will act as a separater:

/* assume this HTML:

<ol class="toc">
	<li>Intro</li>
	<li>Topic
		<ol>
			<li>Subtopic</li>
			<li>Subtopic</li>
			<li>Subtopic</li>
		</ol>
	</li>
	<li>Topic
		<ol>
			<li>Subtopic</li>
			<li>Subtopic</li>
			<li>Subtopic</li>
		</ol>
	</li>
	<li>Closing</li>		
</ol>
*/

ol.toc, ol.toc ol {
	counter-reset: toc;
}
ol li {
	counter-increment: toc;
}
.toc li:before {
	content: "(Item " counters(toc, ".") ")";
}

/* visible page "output"

<ol class="toc">
	<li>(Item 1)Intro</li>
	<li>(Item 2)Topic
		<ol>
			<li>(Item 2.1)Subtopic</li>
			<li>(Item 2.2)Subtopic</li>
			<li>(Item 2.3)Subtopic</li>
		</ol>
	</li>
	<li>(Item 3)Topic
		<ol>
			<li>(Item 3.1)Subtopic</li>
			<li>(Item 3.2)Subtopic</li>
			<li>(Item 3.3)Subtopic</li>
		</ol>
	</li>
	<li>(Item 4)Closing</li>		
</ol>
*/

As you can probably tell, counters could be incredibly helpful when it comes to basic count display up to a table of contents display.

After you get over the initial giggle of the the word counter, you start to realize that they can be quite useful.  In most cases, you'll use counters with :after and :before pseudo-elements, unless you dedicate empty elements to their contents.  I've seen counters used in broadcast slides, slideshows, and documentation.  Do you use counters for anything else?