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<!DOCTYPE html>
<html><head>
<title>CSS Lists and Counters Module Level 3</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" >
<link rel="stylesheet" type="text/css" href="http://dev.w3.org/csswg/default.css">
<style>
p { text-indent: 0 !important; margin: .5em 0; }
code.inline { white-space: nowrap; }
.alphabetic-example { font-family: monospace; font-size: 90%; margin: 1em 0 1em 2em; }
.alphabetic-example > span { display: inline-block; width: 50px;}
.alphabetic-example > span > img { width: 16px; height: 16px; }
pre > code { white-space: pre-wrap; }
</style>
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-ED">
</head><body>
<div class="head">
<!--logo-->
<h1>CSS Lists and Counters Module Level 3</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>This version:
<dd><!--<a href="http://www.w3.org/TR/[YEAR]/WD-[SHORTNAME]-[CDATE]">http://www.w3.org/TR/[YEAR]/WD-[SHORTNAME]-[CDATE]</a></dd>-->
<a href="http://dev.w3.org/csswg/css3-lists/">http://dev.w3.org/csswg/css3-lists/</a></dd>
<dt>Latest version:
<dd><a href="http://www.w3.org/TR/css3-lists/">http://www.w3.org/TR/css3-lists/</a></dd>
<dt>Previous version:</dt>
<dd><a href="http://www.w3.org/TR/2002/WD-css3-lists-20021107/">http://www.w3.org/TR/2002/WD-css3-lists-20021107/</a></dd>
<dt>Editor:</dt>
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>, Google Inc.</dd>
<dt>Previous Editors:</dt> <!-- in the order tantek requested -->
<dd>Ian Hickson, <a href="mailto:ian@hixie.ch">ian@hixie.ch</a></dd>
<dd><span lang="tr">Tantek Çelı̇k</span>, Microsoft Corporation,
<a href="mailto:tantekc@microsoft.com">tantekc@microsoft.com</a></dd>
<dt>Contributors:</dt> <!-- in order of volume of input -->
<dd>Simon Montagu, AOL-TW/Netscape, <a href="mailto:smontagu@netscape.com">smontagu@netscape.com</a></dd>
<dd>Daniel Yacob, <a href="mailto:yacob@geez.org">yacob@geez.org</a></dd>
<dd>Christopher Hoess, <a href="mailto:choess@stwing.upenn.edu">choess@stwing.upenn.edu</a></dd>
<dd>Daniel Glazman, AOL-TW/Netscape, <a href="mailto:glazman@netscape.com">glazman@netscape.com</a></dd>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">
Abstract</h2>
<p>CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc.
This draft contains the features of CSS level 3 relating to
list styling. It includes and extends the functionality of
CSS level 2 [[!CSS21]], which builds on CSS level 1
[[CSS1]]. The main extensions compared to level 2 are a pseudo-element
representing the list marker,
and a method for authors to define their own list-styles.</p>
<h2 class="no-num no-toc" id="status">
Status of this document</h2>
<!--status-->
<h2 class="no-num no-toc" id="contents">
Table of contents</h2>
<!--toc-->
<hr>
<!-- ====================================================================== -->
<h2>
Introduction</h2>
<p>The list model in this module differs in some important ways from the list model in CSS2,
specifically in its handling of markers. Implementation experience suggested the CSS2 model
overloaded the ::before and ::after pseudo-elements with too much behavior, while at the same time
introducing new properties when existing properties were sufficient.</p>
<p>Most block-level elements in CSS generate one principal block box. In this module, we discuss two
CSS mechanisms that cause an element to have an associated marker: one method associates one
principal block box (for the element's content) with a separate marker box (for decoration such as a
bullet, image, or number), and the other inserts a marker box into the principal box. Unlike :before
and :after content, the marker box cannot affect the position of the principal box, whatever the
positioning scheme.</p>
<div class="html-example">
<p>For instance, the following example illustrates how markers may be used to add parentheses around
each numbered list item. This HTML application and style sheet:</p>
<pre>
<!doctype html>
<html>
<head>
<title>Creating a list with markers</title>
<style>
li::marker { content: "(" counter(list-item, lower-roman) ")"; }
li { display: list-item; }
</style>
</head>
<body>
<ol>
<li>This is the first item.</li>
<li>This is the second item.</li>
<li>This is the third item.</li>
</ol>
</body>
</html>
</pre>
<p>should produce something like this: </p>
<pre>
(i) This is the first item.
(ii) This is the second item.
(iii) This is the third item.
</pre>
</div>
<p>With descendant selectors and child selectors, it's possible to specify different marker types
depending on the depth of embedded lists.</p>
<p class="issue">A future release of this module will probably include ways to render tree
lists.</p>
<!-- ====================================================================== -->
<h2>
Declaring a List Item</h2>
<p>To declare a <dfn>list item</dfn>, the 'display'
property must be set to ''list-item'' or ''inline-list-item'' (defined
later in this section). This, in addition to generating a ''::marker''
pseudo-element and enabling the properties described below for that element,
causes that element to increment the list item counter ''list-item''.
(This does not affect the specified or computed values of the counter
properties.)</p>
<p>The ''list-item'' counter is a real counter,
and can be directly affected using the 'counter-increment' and ''counter-reset''
properties. It can also be used in the ''counter()'' and ''counters()'' functions.</p>
<table class="propdef">
<tbody>
<tr>
<th>Property:
<td>'display'
<tr>
<th>New Value:
<td><dfn>inline-list-item</dfn>
<tr>
<th>Initial:
<td>same as CSS2.1
<tr>
<th>Applies to:
<td>same as CSS2.1
<tr>
<th>Inherited:
<td>same as CSS2.1
<tr>
<th>Percentages:
<td>same as CSS2.1
<tr>
<th>Media:
<td>same as CSS2.1
<tr>
<th>Computed value:
<td>same as CSS2.1
</tbody>
</table>
<p>The ''inline-list-item'' display value makes the element a list item,
which means it can generate a ::marker pseudo-element. The element also
affects the predefined ''list-item'' counter, as specified above. Otherwise,
this display value is treated identically to ''inline''.</p>
<p class='note'>This display value is silly, and only necessary because we
haven't yet defined 'display' to be split into subproperties. When that
happens, "being a list item" will just be a property value that can apply
to block, inline, and other elements.</p>
<!-- ====================================================================== -->
<h2 id='marker-content'>
Marker Content: The 'list-style-type' and 'list-style-image' properties</h2>
<table class="propdef">
<tr>
<th>Name:
<td><dfn>list-style-type</dfn>
<tr>
<th>Value:
<td><b><string></b> | <b><counter-style></b> | none
<tr>
<th>Initial:
<td>disc
<tr>
<th>Applies To:
<td><i title="list item">list items</i>
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed Value:
<td>specified value
</table>
<p>When the 'list-style-image' property is ''none'' or not a valid image,
the 'list-style-type' property is instead used to construct the default
contents of a list item's marker.</p>
<dl>
<dt><dfn><string></dfn></dt>
<dd>The ''::marker'' pseudoelement must use the provided string as its default
contents.</dd>
<dt><dfn><counter-style></dfn></dt>
<dd>
<p>The ''::marker'' pseudoelement's default contents must be the value of
the ''list-item'' counter, formatted according to the given counter style.
Algorithms for formatting a value according to a counter style are given
later in this spec.</p>
<p>This specification defines a method for authors to create their
own counter styles which may be used here. Additionally, many
useful counter styles are predefined in the sections on
<a href="#predefined-counters">Complex Counter Styles</a> and
<a href="#ua-stylesheet">Predefined Counter Styles</a>.</p>
</dd>
<dt><dfn>none</dfn></dt>
<dd>The ''::marker'' pseudoelement must have no default contents. This will
suppress the creation of a marker unless the ''::marker'' has its contents
specified directly through the 'content' property.</dd>
</dl>
<table class="propdef">
<tr>
<th>Name:
<td><dfn>list-style-image</dfn>
<tr>
<th>Value:
<td><image> | none
<tr>
<th>Initial
<td>none
<tr>
<th>Applies To:
<td><i title="list item">list items</i>
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed Value:
<td>specified value
</table>
<p>This property sets the image that will be used as the list item marker. When
the <b><image></b> resolves to a valid image, it is used as the default contents
of ''::marker'' instead of the value specified by 'list-style-type'.</p>
<p>If the value ''none'' is provided, or the <b><image></b> doesn't resolve
to a valid image, then the default contents are given by 'list-style-type' instead.</p>
<div class="example">
<p>The following example sets the marker at the beginning of each list item to
be the image "ellipse.png".</p>
<pre>LI { list-style-image: url("http://www.example.com/ellipse.png") }</pre>
</div>
<!-- ====================================================================== -->
<h2 id='list-style-position-property'>
Marker Position: The 'list-style-position' property</h2>
<table class="propdef">
<tr>
<th>Name:
<td><dfn>list-style-position</dfn>
<tr>
<th>Value:
<td>inside | outside
<tr>
<th>Initial:
<td>outside
<tr>
<th>Applies To:
<td><i title="list item">list items</i>
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed Value:
<td>specified value
</table>
<p>This property specifies the position of the ''::marker'' pseudo-element's box
in the <i>list item</i>. Values have the following meanings:</p>
<dl>
<dt><dfn>inside</dfn></dt>
<dd>The ''::marker'' pseudo-element is a 'display:inline' element placed immediately before the ''::before'' pseudo-element in the <i>list item</i>'s principle box, after which the element's content flows. Note that if there is no inline content, this will create a line box, just as content in an inline ''::before'' pseudo-element would. Also note that all the properties that apply to inline elements apply to the ''::marker'' pseudo-element in this state, and this ''::marker'' box participates in the inline box model in the normal manner.</dd>
<dt><dfn>outside</dfn></dt>
<dd>As ''inside'', plus the computed value of 'position' on the ::marker pseudo-element is ''marker''. The section on the new 'marker' value for 'position' explains the consequences of this.</dd>
</dl>
<p>Note that a marker is only generated if the computed value of the 'content'
property for the element's ''::marker'' pseudo-element is not ''none''.</p>
<div class=example>
<p>For example:
<pre class=html-example>
<!doctype html>
<html>
<head>
<title>Comparison of inside/outside position</title>
<style>
ul { list-style: outside; }
ul.compact { list-style: inside; }
</style>
</head>
<body>
<ul>
<li>first list item comes first</li>
<li>second list item comes second</li>
</ul>
<ul class=compact>
<li>first list item comes first</li>
<li>second list item comes second</li>
</ul>
</body>
</html>
</pre>
<p>The above example may be formatted as:</p>
<div class="figure">
<p><img id="img-list-inout" src="list-inout.gif" alt="list with bullets to the left of it and list with bullets inside"></p>
</div>
<p>In right-to-left text, the markers would have been on the right side of the box.</p>
</div>
<!-- ====================================================================== -->
<h2 id='list-style-property'>
The 'list-style' shorthand property</h2>
<table class="propdef">
<tr>
<th>Name:
<td><dfn>list-style</dfn>
<tr>
<th>Value:
<td><'list-style-type'> || <'list-style-position'> || <'list-style-image'>
<tr>
<th>Initial:
<td>see individual properties
<tr>
<th>Applies To:
<td><i title="list item">list items</i>
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed Value:
<td>see individual properties
</table>
<p>The 'list-style' property is a shorthand notation for setting the
three properties 'list-style-type', 'list-style-image', and 'list-style-position'
at the same place in the style sheet.</p>
<div class="example">
<p>For example:</p>
<pre>
UL { list-style: upper-roman inside } /* Any UL */
UL > UL { list-style: circle outside } /* Any UL child of a UL */
</pre>
</div>
<p>Using a value of ''none'' in the shorthand is potentially ambiguous, as ''none'' is a valid value for both 'list-style-image' and 'list-style-type'. To resolve this ambiguity, a value of ''none'' in the shorthand must be applied to whichever of the two properties aren't otherwise set by the shorthand.</p>
<pre class='example css-example'>
list-style: none disc;
/* Sets the image to ''none'' and the type to ''disc''. */
list-style: none url(bullet.png);
/* Sets the image to ''url(bullet.png)'' and the type to ''none''. */
list-style: none;
/* Sets both image and type to ''none''. */
list-style: none disc url(bullet.png);
/* Syntax error */
</pre>
<p>Although authors may specify 'list-style' information directly on
list item elements (e.g., LI in HTML), they should do so with care. The following rules look
similar, but the first declares a descendant selector and the second a (more specific) child selector.</p>
<pre class="example">
OL.alpha LI { list-style: lower-alpha } /* Any LI descendant of an OL */
OL.alpha > LI { list-style: lower-alpha } /* Any LI child of an OL */
</pre>
<p>Authors who use only the descendant selector may not
achieve the results they expect. Consider the following rules:</p>
<pre class="example html-example">
<!doctype html>
<html>
<head>
<title>WARNING: Unexpected results due to cascade</title>
<style>
ol.alpha li { list-style: lower-alpha; }
ul li { list-style: disc; }
</style>
</head>
<body>
<ol class=alpha>
<li>level 1
<ul>
<li>level 2</li>
</ul>
</li>
</ol>
</body>
</html>
</pre>
<p>The desired rendering would have level 1 list items with 'lower-alpha' labels and level 2 items
with 'disc' labels. However, the cascading order will cause the first style rule (which includes
specific class information) to mask the second. The following rules solve the problem by employing a
child selector instead:</p>
<pre class="example">
OL.alpha > LI { list-style: lower-alpha }
UL LI { list-style: disc }
</pre>
<p>Another solution would be to specify 'list-style' information only
on the list type elements:
<pre class="example">
OL.alpha { list-style: lower-alpha }
UL { list-style: disc }
</pre>
<p>Inheritance will transfer the 'list-style' values from OL and UL
elements to LI elements. This is the recommended way to specify list style information.</p>
<!-- ====================================================================== -->
<h2 id='marker-pseudoelement'>
Markers: The ''::marker'' pseudo-element</h2>
<p>Markers are created by setting an element's 'display' property to
''list-item'' or ''inline-list-item''. The marker box is only created if
the computed value of the 'content' property for the pseudo-element is not
''none''.</p>
<p>Just like other generated content, markers generate a box when they're created, which has margins, border, padding, and everything else a box normally has. Markers are placed at the beginning of their superior parent's content, before the ::before pseudo-element (if it exists). Marker boxes are inline by default, though certain values for 'list-style-position' on the list item can make the marker box positioned, which can have an effect on the computed value of display.</p>
<div class="example">
<p>In the following example, the content is centered within a marker
box of a fixed width. This document:</p>
<pre class="html-example">
<!doctype html>
<html>
<head>
<title>Content alignment in the marker box</title>
<style>
LI::marker {
content: "(" counter(counter) ")";
width: 6em;
text-align: center;
}
LI {
display: list-item;
counter-increment: counter;
}
</style>
</head>
<body>
<ol>
<li>This is the first item.</li>
<li>This is the second item.</li>
<li>This is the third item.</li>
</ol>
</body>
</html>
</pre>
<p>should render something like this: </p>
<pre>
(1) This is the
first item.
(2) This is the
second item.
(3) This is the
third item.
</pre>
</div>
<p>The next example uses markers to number notes (paragraphs).</p>
<div class="example">
<p>The following document:</p>
<pre class="html-example">
<!doctype html>
<html>
<head>
<title>Markers to create numbered notes</title>
<style>
P { margin-left: 12 em; }
P.Note::marker {
content: url("note.gif") "Note " counter(note-counter) ":";
text-align: left;
width: 10em;
}
P.Note {
display: list-item;
counter-increment: note-counter;
}
</style>
</head>
<body>
<p>This is the first paragraph in this document.</p>
<p class="Note">This is a very short document.</p>
<p>This is the end.</p>
</body>
</html>
</pre>
<p>should render something like this:</p>
<pre>
This is the first paragraph
in this document.
Note 1: This is a very short
document.
This is the end.
</pre>
</div>
<div class="example">
<p>The following example illustrates how markers may be offset from their element. This HTML
application and style sheet:</p>
<pre class="html-example">
<!doctype html>
<html>
<head>
<title>Marker example</title>
<style>
P { margin-left: 8em } /* Make space for counters */
LI::marker { margin: 0 3em 0 0; content: counter(list-item, lower-roman) "."; }
LI { display: list-item }
</style>
</head>
<body>
<p>This is a long preceding paragraph ...</p>
<ol>
<li>This is the first item.</li>
<li>This is the second item.</li>
<li>This is the third item.</li>
</ol>
<p>This is a long following paragraph ...</p>
</body>
</html>
</pre>
<p>should render something like this: </p>
<pre>
This is a long preceding
paragraph ...
i. This is the first item.
ii. This is the second item.
iii. This is the third item.
This is a long following
paragraph ...
</pre>
<p>(Note the use of the implicit counter increment.)</p>
</div>
<!-- ====================================================================== -->
<h3 id='content-property'>
Generating the computed value of the 'content' property</h3>
<p>If a ''::marker'' pseudo-element has its 'content' property set to ''normal'',
the computed value of the marker's 'content' property must be determined
according to the following algorithm:</p>
<ol>
<li>If the computed value of 'list-style-image' is a valid image, then
the computed value of the 'content' property is that image.</li>
<li>Otherwise, if the computed value of 'list-style-type' is a string, then
the computed value of the 'content' property is that string.</li>
<li>Otherwise, if the computed value of 'list-style-type' is the name of a counter style, then the computed value of the 'content' property is <code><counter-prefix> counter(list-item, <counter-name>) <counter-suffix></code>, where <i><counter-name></i> is the computed value of the 'list-style-type' property and <i><counter-prefix></i> and <i><counter-suffix></i> are the values of the <i title='counter-prefix'>prefix</i> and <i title='counter-suffix'>suffix</i> descriptors for the counter style with that name.</li>
<li>Otherwise the computed value is ''none''.</li>
</ol>
<div class="example">
<p>Given the following style sheet:</p>
<pre>
li { display: list-item; list-style-type: decimal /* initial value */; }
li::marker { content: normal /* initial value */; }
</pre>
<p>And the following document fragment:</p>
<pre>
<li>List Item</li>
</pre>
<p>The computed value of the 'content' property on the ''::marker''
pseudo-element of the list item element is:</p>
<pre>counter(list-item, decimal) "."</pre>
</div>
<!-- ===================================================================== -->
<h2 id='display-marker'>
Positioning Markers: the ''marker'' value for 'position'</h2>
<p>This section introduces a new positioning scheme, designed to model the
way in which "outside" list markers were traditionally positioned in CSS 2.1.
Outside list markers now have their positioning defined in terms of this new
value.</p>
<p>The new positioning scheme defined in this section can be used on all
elements, not just ::marker pseudo-elements. In some situations, such as
legal proceedings or official minutes, the precise form that the list marker
takes is a vital part of the content. It's not acceptable for the marker to
change (from a custom-defined marker to a default bullet or alpha marker)
just because the UA is not rendering CSS, or a server error is temporarily
preventing the CSS file from being loaded, as the precise form of the marker
is used to officially refer to that segment. The only way to guarantee that
the marker will be rendered correctly, regardless of whether CSS is applied,
is to specify the marker outside of CSS, directly in the document's markup.
However, the page author may still want to style the marker in many of the
ways that are available to them when using ordinary CSS-generated markers.
To accomodate this, the new positioning scheme can be used to position the
marker-in-content as if it were an ordinary CSS-generated marker.</p>
<table class="propdef">
<tbody>
<tr>
<th>Property:
<td>'position'
<tr>
<th>New Value:
<td>''marker''
<tr>
<th>Initial:
<td>same as CSS2.1
<tr>
<th>Applies to:
<td>same as CSS2.1
<tr>
<th>Inherited:
<td>same as CSS2.1
<tr>
<th>Percentages:
<td>same as CSS2.1
<tr>
<th>Media:
<td>same as CSS2.1
<tr>
<th>Computed value:
<td>see prose, otherwise same as CSS2.1
</tbody>
</table>
<p>The ''marker'' value for 'position' depends on the element it is set on having a list item ancestor. If the specified value of 'position' is ''marker'' and the element does not have a list item ancestor, 'position' must compute to 'relative' on the element.</p>
<p>The element's vertical position is calculated according to the <a href="http://www.w3.org/TR/CSS21/visuren.html#normal-flow">normal flow</a>. <span class='note'>In the Positioned Layout Module this will be defined more precisely in terms of placeholders.</span></p>
<p>To calculate the element's horizontal position, first define the element's nearest list item ancestor element as the <dfn>ancestor list item</dfn>, and the parent of <em>that</em> element as the <dfn>ancestor list container</dfn>. The element's horizontal position must then be set such that the margin edge of the element corresponding to the "end" edge of the <i>ancestor list container</i> is flush with the border edge of the <i>ancestor list item</i> corresponding to the "start" edge of the <i>ancestor list container</i>.</p>
<p class='note'>The purpose of this somewhat convoluted definition is to position the marker flush against its list item, and to keep all the markers for a given list on the same side of their list items even in mixed-direction text, so that authors can specify padding on only one side and still ensure their markers are visible.</p>
<p class='issue'>This handles ltr/rtl mixed-direction text, but not mixed
horizontal and vertical text. For example, if the <i>ancestor list container</i>
is vertical but the <i>ancestor list item</i> is horizontal, this definition
is nonsensical, as it requires a vertical edge of the marker to be flush
against a horizontal edge of the <i>ancestor list item</i>. Need to think
on what UI would be acceptable for such a crazy case, and how to state it
in non-crazy terms.</p>
<p>The 'top', 'right', 'bottom', and 'left' properties specify offsets relative to the top, right, bottom, and left edges (respectively) of the element itself, similar to how relative positioning works.</p>
<!-- ===================================================================== -->
<h2 id='counter-style'>
Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
<p>CSS 2.1 defined a handful of useful counter styles based on the styles that
HTML traditionally allowed on ordered and unordered lists. This tiny set, though,
is quite inadequate for modern web pages; displaying an ordered list with markers
based on the latin alphabet while the content is Arabic seems quite incongruous!</p>
<p>Unfortunately, the set of potentially useful list styles is too large to
specify ahead of time - the world contains thousands of languages and hundreds
of scripts, not to mention the near-infinite stylistic variations found on the
web that go beyond mere languaged-based variation. The ''@counter-style'' rule
allows CSS to address this in an open-ended manner, by allowing the author
to define their own counter styles. These styles can then be used in the
'list-style-type' property or in the ''counter()'' and ''counters()'' functions,
exactly like the Complex Counter Styles in CSS.</p>
<p>A <dfn>counter style</dfn> defines how to construct the representation of a
counter value. Counter styles are composed of:</p>
<ul>
<li>a <dfn title='counter-name'>name</dfn>, to identify the style</li>
<li>an <dfn title='counter-algorithm'>algorithm</dfn> that transforms counter
values into a string representation</li>
<li>a <dfn title='counter-negative'>negative sign</dfn>, which is prepended or
appended to the representation of a negative counter value</li>
<li>a <dfn title='counter-prefix'>prefix</dfn>, to prepend to the representation</li>
<li>a <dfn title='counter-suffix'>suffix</dfn>, to append to the representation</li>
<li>an <dfn title='counter-upper-bound'>upper</dfn> and
<dfn title='counter-lower-bound'>lower range bound</dfn>, to specify the
(inclusive) range that the counter style can handle</li>
<li>and a <dfn title='counter-fallback'>fallback style</dfn>, to render the
representation with when the counter value is outside the counter style's
range or the counter style otherwise can't render the counter value</li>
</ul>
<p>The algorithm is usually specified implicitly by a combination of the ''type'',
''glyphs'', and ''additive-glyphs'' properties, but some counter styles
instead have their algorithm explicitly defined in the
<a href='#predefined-counters'>Complex Counter Styles</a> section.</p>
<p>When asked to <i title='generate-a-counter'>generate a counter representation</i>
using a particular counter style for a particular counter value, follow these steps:</p>
<ol>
<li>If the counter value is less than the <i title='counter-lower-bound'>lower bound</i>
or greater than the <i title='counter-upper-bound'>upper bound</i> of the counter
style, exit this algorithm and instead <i title='generate-a-counter'>generate a counter representation</i>
using the counter style's fallback style and the same counter value.</li>
<li>If the counter value is negative, set the <var>negative</var> flag and
run the rest of this algorithm with the counter value set to its absolute value instead.</li>
<li>Using the counter value and the <i title='counter-algorithm'>counter algorithm</i>
for the counter style, generate an initial representation for the counter value.</li>
<li>If the <var>negative</var> flag is set, wrap the initial representation in the
counter style's <i title='counter-negative'>negative sign</i>, as specified in
the section for the <a href='#counter-style-negative'>negative descriptor</a>.</li>
<li>Return the representation.</li>
</ol>
<p class='note'>Note that the prefix and suffix don't play a part in this algorithm. This is intentional; the prefix and suffix aren't part of the string returned by the counter() or counters() functions. Instead, the prefix and suffix are added by the algorithm that constructs the computed value of the 'contents' property for the ::marker pseudo-element. This also implies that the prefix and suffix always come from the specified counter-style, even if the actual representation is constructed by a fallback style.</p>
<p>The general form of an ''@counter-style'' rule is:</p>
<pre>
@counter-style <i><counter-style-name></i> {
[ descriptor: value; ]+
}
</pre>
<p>Each @counter-style rule specifies a value for every counter-style descriptor,
either implicitly or explicitly. Those not given explicit value in the rule
take the initial value listed with each descriptor in this specification.
These descriptors apply solely within the ontext of the @counter-style rule
in which they are defined, and do not apply to document language elements.
There is no notion of which elements the descriptors apply to or whether the
values are inherited by child elements. When a given descriptor occurs
multiple times in a given @counter-style rule, only the last specified value
is used; all prior values for that descriptor are ignored.</p>
<p>User agents which do not understand the @counter-style rule encounter the
opening curly bracket and ignore forward until the closing curly bracket.
This at-rule conforms with the forward-compatible parsing requirement of CSS;
parsers may ignore these rules without error. Any descriptors that are not
recognized or implemented by a given user agent must be ignored. @counter-style
rules require a 'type' descriptor; if this is missing the @counter-style is
invalid and must be ignored. The <i><counter-style-name></i> must be be
a valid identifier and must not be "decimal", "default", "inherit", "initial",
"inside", "none", or "outside"; otherwise the @counter-style is invalid and
must be ignored.</p>
<h3 id='counter-style-type'>
Counter algorithms: the 'type' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-type">type</dfn>
<tr>
<th>Value:
<td>repeating | numeric | alphabetic | symbolic | additive
| [non-repeating <integer>?] | [ override <counter-style-name> ]
<tr>
<th>Initial:
<td>N/A
</table>
<p>The 'type' descriptor specifies which algorithm will be used to construct
the counter's representation based on the counter value. For example,
''repeating'' type counter styles just cycle through their glyphs repeatedly,
while ''numeric'' type counter styles interpret their glyphs as digits and
build their representation accordingly. The types are defined as follows:</p>
<h4>
repeating</h4>
<p>If the type is ''repeating'', the 'glyphs' descriptor must contain
at least one <i>counter glyph</i>. This type is defined over all counter
values.</p>
<p>The ''repeating'' counter type cycles repeatedly through its provided
glyphs, looping back to the beginning when it reaches the end of the
list, similar to the default ''disc'' counter style. It can be used for
simple bullets (just provide a single <i>counter glyph</i>), or for cycling
through multiple bullets. The first <i>counter glyph</i> is used as the
representation of the value 1, the second <i>counter glyph</i> (if it exists)
is used as the representation of the value 2, etc.</p>
<p>In general, if there are <var>length</var> <i>counter glyphs</i> and
a representation is being constructed for the value <var>value</var>,
the representation is the <i>counter glyph</i> at index
(<var>value</var> mod <var>length</var>) of the list of <i>counter glyph</i>s
(0-indexed).</p>
<div class=example>
<p>A "triangle bullet" counter style can be defined as:</p>
<pre>
@counter-style <dfn>triangle</dfn> {
type: repeating;
glyphs: '▶';
suffix: '';
}
</pre>
<p>It will then produce lists that look like:</p>
<pre>
▶ One
▶ Two
▶ Three
</pre>
</div>
<h4>
numeric</h4>
<p>If the type is ''numeric'', the 'glyphs' descriptor must contain at
least two <i>counter glyph</i>s. This type is defined over all counter
values.</p>
<p>The ''numeric'' counter type cycles interprets the list of
<i>counter glyph</i>s as digits to a number system, similar to the default
''decimal'' counter style. The first <i>counter glyph</i> in the list is
interpreted as the digit 0, the second as the digit 1, and so on. If there
are <var>length</var> <i>counter glyph</i>s, the representation is a base
<var>length</var> number using the <i>counter glyph</i>s as digits.</p>
<p>To construct the representation, run the following algorithm. Let
<var>length</var> be the length of the list of <i>counter glyphs</i>,
<var>value</var> initially be the counter value, <var>S</var>
initially be the empty string, <var>negative</var> be a boolean flag
that is initially false, and <var>glyph(n)</var> be the nth
<i>counter glyph</i> in the list of <i>counter glyph</i>s (0-indexed).</p>
<ol>
<li>If <var>value</var> is 0, append <var>glyph(0)</var> to
<var>S</var> and return <var>S</var>.</li>
<li>While <var>value</var> is not equal to 0:
<ol>
<li>Prepend <var>glyph( <var>value</var> mod <var>length</var> )</var>
to <var>S</var>.</li>
<li>Set <var>value</var> to <code>floor( <var>value</var> / <var>length</var> )</code>.</li>
</ol>
</li>
<li>Return <var>S</var>.</li>
</ol>
<div class=example>
<p>A "trinary" counter style can be defined as:</p>
<pre>
@counter-style <dfn>trinary</dfn> {
type: numeric;
glyphs: '0' '1' '2';
}
</pre>
<p>It will then produce lists that look like:</p>
<pre>
1. One
2. Two
10. Three
11. Four
12. Five
20. Six
</pre>
</div>
<h4>
alphabetic</h4>
<p>If the type is ''alphabetic'', the 'glyphs' descriptor must contain
at least two <i>counter glyph</i>s. This type is defined only over
positive counter values.</p>
<p>The ''alphabetic'' counter type interprets the list of <i>counter glyphs</i>
as digits to an <em>alphabetic</em> numbering system, similar to the default
''lower-alpha'' counter style. Alphabetic numbering systems are commonly used
for lists, and also appear in many spreadsheet programs to number columns.
The first <i>counter glyph</i> in the list is interpreted as the digit 1,
the second as the digit 2, and so on. If there are <var>length</var>
<i>counter glyph</i>s, the representation is a base <var>length</var> alphabetic
number using the <i>counter glyph</i>s as digits. Alphabetic numbering
systems do not contain a digit representing 0.</p>
<p>To construct the representation, run the following algorithm. Let
<var>length</var> be the length of the list of <i>counter glyph</i>s,
<var>value</var> initially be the counter value, <var>S</var> initially
be the empty string, and <var>glyph(n)</var> be the nth <i>counter glyph</i>
in the list of <i>counter glyph</i>s (0-indexed).</p>
<p>While <var>value</var> is not equal to 0:</p>
<ol>
<li>Set <var>value</var> to <code><var>value</var> - 1</code>.</li>
<li>Prepend <var>glyph( <var>value</var> mod <var>length</var> )</var>