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<!DOCTYPE html>
<html><head>
<title>CSS Counter Styles Level 3</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" >
<link rel="stylesheet" type="text/css" href="../default.css">
<style>
pre.stylesheet { 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 Counter Styles Level 3</h1>
<dl>
<dt>Editor:
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>, Google Inc.
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">
Abstract</h2>
<p>
This module introduces the ''@counter-style'' rule,
which allows authors to define their own custom <i>counter styles</i>
for use with CSS list-marker and generated-content counters [[CSS3LIST]].
It also predefines a set of common counter styles,
including the ones present in CSS2 and CSS2.1.
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.
<h2 class="no-num no-toc" id="status">
Status of this document</h2>
<!--status-->
<p>
The following counter styles are at-risk: ''cjk-decimal'', ''hebrew'', ''hiragana'', ''hiragana-iroha'', ''katakana'', ''katakana-iroha'', ''japanese-informal'', ''japanese-formal'', ''korean-hangul-formal'', ''korean-hanja-informal'', ''korean-hanja-formal'', ''simp-chinese-informal'', ''simp-chinese-formal'', ''trad-chinese-informal'', ''trad-chinese-formal'', and ''ethiopic-numeric''.
<h2 class="no-num no-toc" id="contents">
Table of contents</h2>
<!--toc-->
<hr>
<h2 id='intro'>
Introduction</h2>
<p>
CSS 1 defined a handful of useful counter styles based on the styles that HTML traditionally allowed on ordered and unordered lists.
While this was expanded slightly by CSS2.1,
it doesn't address the needs of worldwide typography.
<p>
This module introduces the ''@counter-style'' rule which 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 <code title='counter-function'>counter()</code> and <code title='counters-function'>counters()</code> functions.
It also defines some additional predefined counter styles,
particularly ones which are common but complicated to represent with ''@counter-style''.
<h2 id='counter-styles'>
Counter Styles</h2>
<p>
A <dfn>counter style</dfn> defines how to convert a counter value into a string.
Counter styles are composed of:
<ul>
<li>
a <dfn title='counter-name'>name</dfn>,
to identify the style
<li>
an <dfn title='counter-algorithm'>algorithm</dfn>
that transforms counter values into a string representation
<li>
a <dfn title='counter-negative'>negative sign</dfn>,
which is prepended or appended to the representation of a negative counter value
<li>
a <dfn title='counter-prefix'>prefix</dfn>,
to prepend to the representation
<li>
a <dfn title='counter-suffix'>suffix</dfn>,
to append to the representation
<li>
a <dfn title='counter-range'>range</dfn>,
which limits the values that a counter style handles
<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
</ul>
<p>
When asked to <dfn title='generate-a-counter'>generate a counter representation</dfn>
using a particular counter style for a particular counter value,
follow these steps:
<ol>
<li>
If the counter value is outside the <i title='counter-range'>range</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>
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>
Return the representation.
</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 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.
<h2 id='the-counter-style-rule'>
Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
<p>
The <dfn id='at-counter-style'>''@counter-style''</dfn> rule allows authors to define a custom <i>counter style</i>.
The components of a <i>counter style</i> are specified by descriptors in the ''@counter-style'' rule.
The <i title='counter-algorithm'>algorithm</i> is specified implicitly by a combination of the ''type'', ''symbols'', and ''additive-symbols'' properties.
<p>
The general form of an ''@counter-style'' rule is:
<pre>
@counter-style <i><counter-style-name></i> {
[ descriptor: value; ]+
}
</pre>
<p class='issue'>
This isn't strictly correct.
For example, multiple semicolons in a row are valid.
This should be done with some productions from the Syntax spec,
so I can just say <code><declaration-list></code> or something.
<p>
The <i><counter-style-name></i> must be be a valid identifier
and must not be ''decimal'' or ''none'',
or else the rule is invalid.
(Note, however, that some names, like ''inside'' or ''initial'',
might conflict with the existing values of properties like 'list-style',
and thus won't be usable there.)
<p class='issue'>
The names should be case-insensitive,
but we haven't yet figured out exactly which kind of insensitivity we want for CSS in general.
<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 context 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 must be ignored.
<p>
Defining a ''@counter-style'' makes it available to the entire document in which it is included.
If multiple ''@counter-style'' rules are defined with the same name,
only one wins,
according to standard cascade rules.
''@counter-style'' rules cascade "atomically":
if one replaces another of the same name,
it replaces it <em>entirely</em>,
rather than just replacing the specific descriptors it specifies.
<p>
This at-rule conforms with the forward-compatible parsing requirement of CSS;
conformant parsers that don't understand these rules will ignore them without error.
Any descriptors that are not recognized or implemented by a given user agent must be ignored in their entirety;
they do not make the ''@counter-style'' rule invalid.
<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 | <span class=nobr>[fixed <integer>?]</span> | <span class=nobr>[ override <counter-style-name> ]</span>
<tr>
<th>Initial:
<td>symbolic
</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 symbols repeatedly,
while ''numeric'' type counter styles interpret their symbols as digits and
build their representation accordingly. The types are defined as follows:
<h4>
Cycling Symbols: the ''repeating'' type</h4>
<p>
The ''repeating'' counter type cycles repeatedly through its provided symbols,
looping back to the beginning when it reaches the end of the list.
It can be used for simple bullets
(just provide a single <i>counter symbol</i>),
or for cycling through multiple symbols.
The first <i>counter symbol</i> is used as the representation of the value 1,
the second <i>counter symbol</i> (if it exists) is used as the representation of the value 2, etc.
<p>
If the type is <dfn>''repeating''</dfn>,
the 'symbols' descriptor must contain at least one <i>counter symbol</i>,
or else the ''@counter-style'' rule is invalid.
This type is defined over all counter values.
<div class=example>
<p>A "triangle bullet" counter style can be defined as:
<pre>
@counter-style <dfn>triangle</dfn> {
type: repeating;
symbols: ‣;
suffix: '';
}
</pre>
<p>It will then produce lists that look like:
<pre>
‣ One
‣ Two
‣ Three
</pre>
</div>
<p>
If there are <var>N</var> <i>counter symbols</i>
and a representation is being constructed for the integer <var>value</var>,
the representation is the <i>counter symbol</i>
at index ( (<var>value</var>-1) mod <var>N</var>)
of the list of <i>counter symbols</i> (0-indexed).
<h4>
Positional Numerals: the ''numeric'' type</h4>
<p>
The ''numeric'' counter type cycles interprets the list of <i>counter symbols</i> as digits to a positional numbering system,
similar to the default <i>decimal</i> counter style.
The first <i>counter symbol</i> in the list is interpreted as the digit 0,
the second as the digit 1,
and so on.
<p>
If the type is <dfn>''numeric''</dfn>,
the 'symbols' descriptor must contain at least two <i>counter symbols</i>,
or else the ''@counter-style'' rule is invalid.
This type is defined over all counter values.
<div class=example>
<p>A "trinary" counter style can be defined as:
<pre>
@counter-style <dfn>trinary</dfn> {
type: numeric;
symbols: '0' '1' '2';
}
</pre>
<p>It will then produce lists that look like:
<pre>
1. One
2. Two
10. Three
11. Four
12. Five
20. Six
</pre>
</div>
<p>
If there are <var>N</var> <i>counter symbols</i>,
the representation is a base <var>N</var> number using the <i>counter symbols</i> as digits.
To construct the representation, run the following algorithm:
<p>
Let <var>N</var> be the length of the list of <i>counter symbols</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>symbol(n)</var> be the nth <i>counter symbol</i> in the list of <i>counter symbols</i> (0-indexed).
<ol>
<li>If <var>value</var> is 0, append <var>symbol(0)</var> to
<var>S</var> and return <var>S</var>.
<li>If <var>value</var> is negative, run the next step of this algorithm with <var>value</var> being the absolute value of <var>value</var> instead.
<li>While <var>value</var> is not equal to 0:
<ol>
<li>Prepend <var>symbol( <var>value</var> mod <var>N</var> )</var>
to <var>S</var>.
<li>Set <var>value</var> to <code>floor( <var>value</var> / <var>N</var> )</code>.
</ol>
<li>If <var>value</var> was originally negative, wrap <var>S</var> 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>Return <var>S</var>.
</ol>
<h4>
Bijective Numerals: the ''alphabetic'' type</h4>
<p>
The ''alphabetic'' counter type interprets the list of <i>counter symbols</i> as digits to an <em title=''>alphabetic</em> numbering system,
similar to the default ''lower-alpha'' counter style,
which wraps from 'a', 'b', 'c', to 'aa', 'ab', 'ac'.
Alphabetic numbering systems do not contain a digit representing 0;
so the first value when a new digit is added is composed solely of the first digit.
Alphabetic numbering systems are commonly used for lists,
and also appear in many spreadsheet programs to number columns.
The first <i>counter symbol</i> in the list is interpreted as the digit 1,
the second as the digit 2,
and so on.
<p>
If the type is <dfn>''alphabetic''</dfn>,
the 'symbols' descriptor must contain at least two <i>counter symbols</i>,
or else the ''@counter-style'' rule is invalid.
This type is defined only over strictly positive counter values.
<div class=example>
<p>A counter style using go stones can be defined as:
<pre>
@counter-style <dfn>go</dfn> {
type: alphabetic;
symbols: url(white.svg) url(black.svg);
suffix: '';
}
</pre>
<p>It will then produce lists that look like:
<div class='alphabetic-example'>
<span><img src=images/white.svg alt=''></span> One<br>
<span>&l
12C8
t;img src=images/black.svg alt=''></span> Two<br>
<span><img src=images/white.svg alt=''><img src=images/white.svg alt=''></span> Three<br>
<span><img src=images/white.svg alt=''><img src=images/black.svg alt=''></span> Four<br>
<span><img src=images/black.svg alt=''><img src=images/white.svg alt=''></span> Five<br>
<span><img src=images/black.svg alt=''><img src=images/black.svg alt=''></span> Six<br>
<span><img src=images/white.svg alt=''><img src=images/white.svg alt=''><img src=images/white.svg alt=''></span> Seven
</div>
<p class='note'>This example requires support for SVG images to display
correctly.
</div>
<div class='example'>
<p>Alphabetic styles may also be used to simulate a fixed-width numeric
style:
<pre>
@counter-style <dfn>fixed-decimal</dfn> {
type: alphabetic;
symbols: '0' '1' '2' '3' '4' '5' '6' '7' '8' '9';
}
ol {
list-style: fixed-decimal;
counter-reset: list-item 1111;
}
</pre>
<p>This will produce lists that look like:
<pre>
0001. One
0002. Two
0003. Three
0004. Four
0005. Five
0006. Six
</pre>
<p>Two-digit numbers start at value 11, three-digit numbers start at
value 111, etc..
<p class='issue'>Should I instead explicitly provide a fixed-width numeric
counter type? I'd like to see if this sort of numbering is used in the
wild first.
</div>
<p>
If there are <var>N</var> <i>counter symbols</i>,
the representation is a base <var>N</var> alphabetic number using the <i>counter symbols</i> as digits.
To construct the representation, run the following algorithm:
<p>
Let <var>N</var> be the length of the list of <i>counter symbols</i>,
<var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol(n)</var> be the nth <i>counter symbol</i> in the list of <i>counter symbols</i> (0-indexed).
<p>While <var>value</var> is not equal to 0:
<ol>
<li>Set <var>value</var> to <code><var>value</var> - 1</code>.
<li>Prepend <var>symbol( <var>value</var> mod <var>N</var> )</var>
to <var>S</var>.
<li>Set <var>value</var> to <code>floor( <var>value</var> / <var>N</var> )</code>.
</ol>
<p>Finally, return <var>S</var>.
<h4>
Repeating Symbols: the ''symbolic'' type</h4>
<p>
The ''symbolic'' counter type cycles repeatedly through its provided symbols,
doubling, tripling, etc. the symbols on each successive pass through the list.
For example, if the original symbols were "*" and "†",
then on the second pass they would instead be "**" and "††",
while on the third they would be "***"and "†††", etc.
It can be used for footnote-style markers,
and is also sometimes used for alphabetic-style lists for a slightly different presentation than what the ''alphabetic'' type presents.
<p>
If the type is <dfn>''symbolic''</dfn>,
the 'symbols' descriptor must contain at least one <i>counter symbol</i>,
or else the ''@counter-style'' rule is invalid.
This type is defined only over strictly positive counter values.
<div class=example>
<p>An "footnote" counter style can be defined as:
<pre>
@counter-style <dfn>footnote</dfn> {
type: symbolic;
symbols: * ⁑ † ‡;
suffix: '';
}
</pre>
<p>It will then produce lists that look like:
<pre>
*. One
⁑. Two
†. Three
‡. Four
**. Five
⁑⁑. Six
</pre>
</div>
<div class=example>
<p>
Some style guides mandate a list numbering that looks similar to ''upper-alpha'',
but repeats differently after the first 26 values,
instead going "AA", "BB", "CC", etc.
This can be achieved with the symbolic type:
<pre>
@counter-style <dfn>upper-alpha-legal</dfn> {
type: symbolic;
symbols: A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z;
}</pre>
<p>
This style is identical to ''upper-alpha'' through the first 27 values,
but they diverge after that, with ''upper-alpha'' going "AB", "AC", "AD", etc.
Starting at the 53rd value, ''upper-alpha'' goes "BA", "BB", "BC", etc.,
while this style jumps into triple digits with "AAA", "BBB", "CCC", etc.
</div>
<p>
To construct the representation, run the following algorithm:
<p>
Let <var>N</var> be the length of the list of <i>counter symbols</i>,
<var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol(n)</var> be the nth <i>counter symbol</i> in the list of <i>counter symbols</i> (0-indexed).
<ol>
<li>Let the <var>chosen symbol</var> be <code class='inline'>symbol(<var>value</var> mod <var>N</var>)</code>.
<li>Let the <var>representation length</var> be <code class='inline'>floor( (<var>value</var> - 1) / <var>N</var> )</code>.
<li>Append the <var>chosen symbol</var> to <var>S</var> a number of times equal to the <var>representation length</var>.
</ol>
<p>Finally, return <var>S</var>.
<p>
The ''symbolic'' type will produce representations with sizes that are linear in the magnitude of the counter value.
This can potentially be abused to generate excessively large representations
and consume undue amounts of the user's memory or even hang their browser.
User agents must support representations at least 20 characters long,
but they may choose to instead use the fallback style for representations that would be longer than 20 characters.
<h4>
Exhaustible Symbols: the ''fixed'' type</h4>
<p>
The <dfn>''fixed''</dfn> counter type runs through its list of counter symbols once,
then falls back.
It is useful for representing counter styles that only have a finite number of representations.
For example, Unicode defines several limited-length runs of special characters meant for lists,
such as circled digits.
<p>
If the type is ''fixed'',
the 'symbols' descriptor must contain at least one <i>counter symbol</i>,
or else the ''@counter-style'' rule is invalid.
This type is defined over counter values in a finite range,
starting with the <i>first symbol value</i> and having a length equal to the length of the list of <i>counter symbols</i>.
<p>
When this type is specified,
it may optionally have an integer provided after it,
which sets the <dfn>first symbol value</dfn>.
If it is omitted, the <i>first symbol value</i> is 1.
<div class=example>
<p>A "box-corner" counter style can be defined as:
<pre>
@counter-style <dfn>box-corner</dfn> {
type: fixed;
symbols: ◰ ◳ ◲ ◱;
suffix: ':';
}
</pre>
<p>It will then produce lists that look like:
<pre>
◰: One
◳: Two
◲: Three
◱: Four
5: Five
6: Six
</pre>
</div>
<p>
The first <i>counter symbol</i> is the representation for the <i>first symbol value</i>,
and subsequent counter values are represented by subsequent <i>counter symbols</i>.
Once the list of <i>counter symbols</i> is exhausted,
further values cannot be represented by this type,
and must instead be represented by the fallback counter style.
<h4>
Accumulating Numerals: the ''additive'' type</h4>
<p>
The <dfn>''additive''</dfn> counter type is used to represent "sign-value" numbering systems,
which, rather than using reusing digits in different positions to change their value,
simply define additional digits with much larger values,
so that the value of the number can be obtained by adding all the digits together.
This is used in Roman numerals
and other numbering systems around the world.
<p>
If the type is ''additive'',
the 'additive-symbols' descriptor must contain at least one <i>additive tuple</i>,
or else the ''@counter-style'' rule is invalid.
This type is nominally defined over all counter values
(see algorithm, below, for exact details).
<div class=example>
<p>A "dice" counter style can be defined as:
<pre>
@counter-style <dfn>dice</dfn> {
type: additive;
additive-symbols: 6 ⚅, 5 ⚄, 4 ⚃, 3 ⚂, 2 ⚁, 1 ⚀;
suffix: '';
}
</pre>
<p>It will then produce lists that look like:
<pre>
⚀ One
⚁ Two
⚂ Three
...
⚅⚄ Eleven
⚅⚅ Twelve
⚅⚅⚀ Thirteen
</pre>
</div>
<p>
To construct the representation, run this algorithm:
<p>
Let <var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol list</var> initially be the list of <i>additive tuples</i>.
<ol>
<li>If <var>value</var> is initially 0, and there is an <i>additive tuple</i> with a weight of 0, append that tuple's <i>counter symbol</i> to S and return S.
<li>If <var>value</var> is negative, run the next step of this algorithm with <var>value</var> being the absolute value of <var>value</var> instead.
<li>While <var>value</var> is greater than 0 and there are elements left in the <var>symbol list</var>:
<ol>
<li>Pop the first <i>additive tuple</i> from the <var>symbol list</var>.
This is the <dfn title="current tuple|current tuple's">current tuple</dfn>.
<li>Append the <i>current tuple</i>'s <i>counter symbol</i> to <var>S</var>
<code>floor( <var>value</var> / <var><i>current tuple's</i> weight</var> )</code>
times (this may be 0).
<li>Decrement <var>value</var> by the <var><i>current tuple's</i> weight multiplied
by the number of times the <i>current tuple</i> was appended to <var>S</var>
in the previous step.
</ol>
<li>If <var>value</var> was originally negative, wrap <var>S</var> 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>If the loop ended because <var>value</var> is 0, return S. Otherwise, the given counter value cannot be represented by this counter style, and must instead be represented by the fallback counter style.
</ol>
<p>
The ''additive'' type will produce representations with sizes that are linear in the magnitude of the counter value.
This can potentially be abused to generate excessively large representations and consume undue amounts of the user's memory or even hang their browser.
User agents must support representations at least 20 characters long,
but they may choose to instead use the fallback style for representations that would be longer than 20 characters.
<h4>
Tweaking Existing Counter Styles: the ''override'' type</h4>
<p>
The <dfn>override</dfn> type allows an author to use the <i>algorithm</i> of another counter style,
but alter other aspects,
such as the negative sign or the suffix.
If a counter style uses the <i>override</i> type,
any unspecified descriptors must be taken from the overridden counter style specified,
rather than taking their initial values.
<p>
If a @counter-style uses the <i>override</i> type,
it must not contain a 'symbols' or 'additive-symbols' descriptor,
or else the ''@counter-style'' rule is invalid.
If the specified counter style name isn't the name of any currently-defined counter style,
it must be treated as if it was overriding the <i>decimal</i> counter style.
<h3 id='counter-style-negative'>
Formatting negative values: the 'negative' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-negative">negative</dfn>
<tr>
<th>Value:
<td><string> <string>?
<tr>
<th>Initial:
<td>"\2D" ("-" hyphen-minus)
</table>
<p>The 'negative' descriptor defines how to alter the representation when
the counter value is negative. Not all counter types can render negative
numbers.
<p>The first string in the value is prepended to the representation when the
counter value is negative. The second string, if specified, is appended
to the representation when the counter value is negative.
<div class='example'>
<p>For example, specifying ''negative: "(" ")";'' will make negative
values be wrapped in parentheses, which is sometimes used in financial
contexts, like "(2) (1) 0 1 2 3...".
</div>
<h3 id='counter-style-prefix'>
Symbols before the marker: the 'prefix' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-prefix">prefix</dfn>
<tr>
<th>Value:
<td><string>
<tr>
<th>Initial:
<td>"" (the empty string)
</table>
<p>The 'prefix' descripter specifies a string that is prepended to the
marker representation. Prefixes are only added by the algorithm for constructing
the default contents of the ''::marker'' pseudo-element; the prefix is not
added automatically when the counter() or counters() functions are used.
Prefixes are added to the representation after negative signs.
<h3 id='counter-style-suffix'>
Symbols after the marker: the 'suffix' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-suffix">suffix</dfn>
<tr>
<th>Value:
<td><string>
<tr>
<th>Initial:
<td>"\2E" ("." full stop)
9DFA
</table>
<p>The 'suffix' descripter specifies a string that is appended to the
marker representation. Suffixes are only added by the algorithm for constructing
the default contents of the ''::marker'' pseudo-element; the suffix is not
added automatically when the counter() or counters() functions are used.
Suffixes are added to the representation after negative signs.
<h3 id='counter-style-range'>
Limiting the counter scope: the 'range' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-range">range</dfn>
<tr>
<th>Value:
<td>[ [ <integer> | infinite ]{2} ]# | auto
<tr>
<th>Initial:
<td>auto
</table>
<p>The 'range' descriptor defines the ranges over which the counter style is defined.
If a counter style is used to represent a counter value outside of its ranges,
the counter style instead drops down to its fallback counter style.
<p>If the value is not ''auto'', the first value of each range in the list represents the lower bound of the range (with ''infinite'' representing negative infinity), and the second value represents the upper bound of the range (with 'infinite' representing positive infinity). This is an inclusive range - it includes both the lower and upper bound numbers. The range of the counter style as a whole is the union of the individual ranges. If the lower bound of any range is higher than the higher bound, the entire descriptor is invalid and must be ignored.
<p>If the value is ''auto'', the range depends on the counter type. For ''repeating'', ''numeric'', and ''fixed'' types, it must be treated identically to specifying ''infinite infinite''. For ''alphabetic'' and ''symbolic'', it must be treated identically to ''1 infinite''. For ''additive'', it must be treated identically to ''0 infinite''. For ''override'', it must be treated according to the type of the counter style it is overriding.
<p>Some counter style types have their own implicit ranges, specified above in the individual descriptions for each type. The explicit range given by the ''<code class="css">range</code>'' descriptor applies at the same time as the implicit range given by the ''<code class="css">type</code>'' descriptor - if the counter value is outside either range, the fallback style must instead be used to generate the representation.
<p class=issue>There's also an implicit range coming from implementation
limits. Should we require UAs to support all values in a signed 2-byte int,
or a signed 4-byte int?
<h3 id='counter-style-fallback'>
Defining fallback: the 'fallback' descriptor</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-fallback">fallback</dfn>
<tr>
<th>Value:
<td><counter-style-name>
<tr>
<th>Initial:
<td>decimal
</table>
<p>The 'fallback' descriptor specifies a fallback counter style to be used
when the current counter style can't create a representation for a given
counter value. For example, if a counter style defined with a range of 1-10
is asked to represent a counter value of 11, the counter value's representation
is instead constructed with the fallback counter style (or possibly the
fallback style's fallback style, if the fallback style can't represent that
value, etc.).
<p>If the value of the 'fallback' descriptor isn't the name of any
currently-defined counter style, the used value of the 'fallback' descriptor
is <i>decimal</i> instead. Similarly, while following fallbacks to find
a counter style that can render the given counter value, if a loop in the
specified fallbacks is detected, the <i>decimal</i> style must be used instead.
<p class='note'>Note that it is not necessarily an error to specify fallback
loops. For example, if an author desires a counter style with significantly
different representations for even and odd counter values, they may find it
easiest to define one style that can only represent odd values and one that
can only represent even values, and specify each as the fallback for the
other one. Though the fallback graph is circular, at no point do you encounter
a loop while following these fallbacks - every counter value is represented
by one or the other counter style.
<p class='issue'>Is it useful to allow
this case? If it would be significantly easier for implementations to just
detect and reject circular fallback graphs, that would probably be acceptable.</span>
<h3 id='counter-style-symbols'>
Marker characters: the 'symbols' and 'additive-symbols' descriptors</h3>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-symbols">symbols</dfn>
<tr>
<th>Value:
<td>[ <string> | <image> | <identifier> ]+
<tr>
<th>Initial:
<td>N/A
</table>
<table class='descdef'>
<tr>
<th>Name:
<td><dfn id="descdef-additive-symbols">additive-symbols</dfn>
<tr>
<th>Value:
<td>[ <integer> && [ <string> | <image> | <identifier> ] ]#
<tr>
<th>Initial:
<td>N/A
</table>
<p>The 'symbols' and 'additive-symbols' descriptors specify the symbols
used by the marker-construction algorithm specified by the 'type' descriptor.
The 'symbols' descriptor must be specified if the counter type is ''repeating'',
''numeric'', ''alphabetic'', ''symbolic'', or ''fixed'', and the
'additive-symbols' descriptor must be specified if the counter type is
''additive''; otherwise, the @counter-style is invalid and must be ignored.
<p>Some counter styles specify that the 'symbols' descriptor must have at
least two entries. If the counter's style is such a type, and the 'symbols'
descriptor has only a single entry, the counter style is invalid and must
be ignored.
<p>Each entry in the 'symbols' descriptor's value defines a <dfn>counter symbol</dfn>,
which is interpreted differently based on the counter style's type. Each
entry in the 'additive-symbols' descriptor's value defines an <dfn>additive tuple</dfn>,
which consists of a <i>counter symbol</i> and a non-negative integer weight.
Each weight must be a non-negative integer, and the <i>additive tuples</i>
must be specified in order of descending weight; otherwise, the @counter-style
is invalid and must be ignored.
<p>Counter symbols may be strings, images, or identifiers, and the three types can be mixed in a single descriptor. Counter representations are constructed by concatenating counter symbols together. Identifiers are rendered as strings containing the same characters. Images are rendered as inline replaced elements. The <a href="http://dev.w3.org/csswg/css3-images/#default-object-size">default object size</a> of an image counter symbol is a 1em by 1em square.
<!-- ====================================================================== -->
<h2 id='symbols-function'>
Defining Anonymous Counter Styles: the ''symbols()'' function</h2>
<p>
The previous chapter specified a way to define custom counter styles.
However, counter styles are sometimes used only once in a stylesheet,
and defining a full ''@counter-style'' rule can be overkill for this case
(not to mention the possibility of unintentional name collisions).
To address this case,
the ''symbols()'' function provides a simple way to define an anonymous counter style as an inline value.
It does not provide the full feature-set of the ''@counter-style'' rule,
but provides a sufficient subset to still be useful.
The syntax of the ''symbols()'' rule is:
<pre><dfn id='type-symbols-function'><symbols-function></dfn> = symbols( <symbols-type>? [ <string> | <image> ]+ )</pre>
<p>
Where <symbols-type> is one of the following keywords:
''repeating'', ''numeric'', ''alphabetic'', ''symbolic'', or ''fixed''.
<p>
The ''symbols()'' function defines an anonymous counter style
with no <i title="counter-name">name</i>,
a <i title="counter-prefix">prefix</i>
and <i title="counter-suffix">suffix</i> of ''""'' (the empty string),
a <i title="counter-range">range</i> from negative infinity to positive infinity,
n <i title="counter-fallback">fallback style</i> of ''decimal'',
and a <i title="counter-negative">negative sign</i> of "\2D" ("-" hyphen-minus).
The counter style's <i title="counter-algorithm">algorithm</i> is constructed
by consulting the previous chapter using the provided type
- or ''symbolic'' if the type was omitted -
and the provided <string>s and <image>s as the value of the 'symbols' property.
If the type is ''fixed'', the <i>first symbol value</i> is ''1''.
<div class='example'>
<p>
This code:
<pre>ol { list-style: symbols("*" "\2020" "\2021" "\A7"); }</pre>
<p>
will produce lists that look like:
<pre>
* One
† Two
‡ Three
§ Four
** Five
†† Six
‡‡ Seven</pre>
<p>
On the other hand, specifying the type of counter, like so:
<pre>ol { list-style: symbols(repeating "*" "\2020" "\2021" "\A7"); }</pre>
<p>
will produce lists that look like:
<pre>
* One
† Two
‡ Three
§ Four
* Five
† Six
‡ Seven</pre>
</div>
<p class='note'>
Note that the ''symbols()'' function only allows strings and images,
while the 'symbols' descriptor of a ''@counter-style'' rule also allows identifiers.
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<h2 id='predefined-counters'>
Simple Predefined Counter Styles</h2>
<p>
The following stylesheet uses the ''@counter-style'' rule
to redefine all of the counter styles defined in CSS 2 and CSS 2.1.
This stylesheet is normative -
UAs must include it in their UA stylesheet