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<pre class='metadata'>
Title: CSS Lists Module Level 3
Shortname: css-lists
Level: 3
Group: CSSWG
Status: ED
Work Status: Refining
ED: https://drafts.csswg.org/css-lists-3/
TR: https://www.w3.org/TR/css-lists-3/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Ian Hickson, Google, ian@hixie.ch
Former Editor: Tantek Çelı̇k, Formerly of Microsoft, tantekc@microsoft.com
Previous Version: https://www.w3.org/TR/2019/WD-css-lists-3-20190425/
Previous Version: https://www.w3.org/TR/2014/WD-css-lists-3-20140320/
Previous Version: https://www.w3.org/TR/2011/WD-css3-lists-20110524/
!Contributors: Simon Montagu, AOL-TW/Netscape, <a href="mailto:smontagu@netscape.com">smontagu@netscape.com</a>
!Contributors: Daniel Yacob, <a href="mailto:yacob@geez.org">yacob@geez.org</a>
!Contributors: Christopher Hoess, <a href="mailto:choess@stwing.upenn.edu">choess@stwing.upenn.edu</a>
!Contributors: Daniel Glazman, AOL-TW/Netscape, <a href="mailto:glazman@netscape.com">glazman@netscape.com</a>
Abstract: This module contains CSS features related to list counters: styling them, positioning them, and manipulating their value.
</pre>
<pre class='link-defaults'>
spec:css-pseudo-4; type:selector; text:::before
spec:infra; type:dfn;
text:list
text:string
</pre>
<h2 id='intro'>
Introduction</h2>
This specification defines the ''::marker'' pseudo-element,
the ''list-item'' <a>display type</a> that generates markers,
and several properties controlling the placement and styling of markers.
It also defines <a>counters</a>,
which are special numerical objects
often used to generate the default contents of markers.
<div class="example">
For instance, the following example illustrates
how markers can be used to add parentheses around each numbered list item:
<xmp highlight=html>
<style>
li::marker { content: "(" counter(list-item, lower-roman) ")"; }
li { display: list-item; }
</style>
<ol>
<li>This is the first item.
<li>This is the second item.
<li>This is the third item.
</ol>
</xmp>
It should produce something like this:
<pre>
(i) This is the first item.
(ii) This is the second item.
(iii) This is the third item.
</pre>
Note: Note that this example is far more verbose than is usually needed in HTML,
as the UA default style sheet takes care of most of the necessary styling.
</div>
With descendant selectors and child selectors,
it's possible to specify different marker types depending on the depth of embedded lists.
<h2 id='declaring-a-list-item'>
Declaring a List Item</h2>
A <dfn export>list item</dfn> is any element with its 'display' property set to ''display/list-item''.
<a>List items</a> generate ''::marker'' pseudo-elements;
no other elements do.
Additionally, <a>list items</a> automatically increment an implied ''list-item'' <a>counter</a>
(see [[#list-item-counter]]).
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<h2 id='markers'>
Markers</h2>
The defining feature of the <a>list item</a> <a>display type</a>
is its <dfn lt="marker | marker box" export>marker</dfn>,
a symbol or ordinal that helps denote the beginning of each <a>list item</a> in a list.
In the CSS layout model,
<a>list item</a> <a>markers</a> are represented by
a <a>marker box</a> associated with each <a>list item</a>.
The contents of this <a>marker</a> can be controlled with
the 'list-style-type' and 'list-style-image' properties on the <a>list item</a>
and by assigning properties to its ''::marker'' pseudo-element.
<h3 id='marker-pseudo'>
The ''::marker'' Pseudo-Element</h3>
The <a>marker box</a> is generated by
the ''::marker'' pseudo-element of a <a>list item</a>
as the [=list item’s=] first child,
before the ''::before'' pseudo-element
(if it exists on the element).
It is filled with content
as defined in [[#content-property]].
<a>Marker boxes</a> only exist for <a>list items</a>:
on any other element,
the ''::marker'' pseudo-element's 'content' property must compute to ''content/none'',
which suppresses its creation.
Only a limited set of properties can be used on the ''::marker'' pseudo-element.
This list is defined in [[css-pseudo-4#marker-pseudo]].
<div class="example">
In this example, markers are used to number paragraphs that are designated as "notes":
<xmp highlight=html>
<style>
p { margin-left: 12 em; }
p.note {
display: list-item;
counter-increment: note-counter;
}
p.note::marker {
content: "Note " counter(note-counter) ":";
}
</style>
<p>This is the first paragraph in this document.
<p class="note">This is a very short document.
<p>This is the end.
</xmp>
It should render something like this:
<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">
By using the ''::marker'' pseudo-element,
a list's markers can be styled
independently from the text of the list item itself:
<xmp highlight=html>
<style>
p { margin-left: 8em } /* Make space for counters */
li { list-style-type: lower-roman; }
li::marker { color: blue; font-weight:bold; }
</style>
<p>This is a long preceding paragraph ...
<ol>
<li>This is the first item.
<li>This is the second item.
<li>This is the third item.
</ol>
<p>This is a long following paragraph ...
</xmp>
The preceding document should render something like this:
<pre>
This is a long preceding
paragraph ...
<span style=color:blue;font-weight:bold;>i.</span> This is the first item.
<span style=color:blue;font-weight:bold;>ii.</span> This is the second item.
<span style=color:blue;font-weight:bold;>iii.</span> This is the third item.
This is a long following
paragraph ...
</pre>
Previously the only way to style a marker was through inheritance;
one had to put the desired marker styling on the list item,
and then revert that on a wrapper element around the list item's actual contents.
</div>
<h3 id='content-property'>
Generating Marker Contents</h3>
The contents of a <a>marker box</a>
are determined by the first of these conditions that is true:
<dl class=switch>
<dt>'content' on the ''::marker'' itself is not ''content/normal''
<dd>
The contents of the <a>marker box</a>
are determined as defined by the 'content' property,
exactly as for ''::before''.
<dt>'list-style-image' on the [=originating element=] defines a [=marker image=]
<dd>
The '<a>marker box</a> contains
an <a>anonymous</a> <a>inline</a> <a>replaced element</a>
representing the specified [=marker image=],
followed by a <a>text run</a> consisting of a single space (U+0020 SPACE).
<dt>'list-style-type' on the [=originating element=] defines a [=marker string=]
<dd>
The <a>marker box</a> contains
a [=text run=] consisting of the specified [=marker string=].
<dt>otherwise
<dd>
The <a>marker box</a> has no contents
and ''::marker'' does not generate a box.
</dl>
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<h3 id='image-markers'>
Image Markers: the 'list-style-image' property</h3>
<pre class="propdef">
Name: list-style-image
Value: <<image>> | none
Initial: none
Applies to: <a>list items</a>
Inherited: yes
Percentages: n/a
Computed value: the keyword ''list-style-image/none''or the computed <<image>>
Animation type: discrete
</pre>
Specifies the <dfn>marker image</dfn>,
which is used to fill the [=list item’s=] [=marker=]
when its 'content' is ''content/normal''.
The values are as follows:
<dl dfn-type="value" dfn-for="list-style-image">
<dt><dfn><<image>></dfn>
<dd>
If the <<image>> represents a [=valid image=],
specifies the element's <a>marker image</a> as the <<image>>.
Otherwise,
the element has no [=marker image=].
<dt><dfn>none</dfn>
<dd>
The element has no <a>marker image</a>.
</dl>
<div class="example">
The following example sets the marker at the beginning of each list item to
be the image "ellipse.png".
<pre highlight=css>li { list-style-image: url("http://www.example.com/ellipse.png") }</pre>
</div>
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<h3 id='text-markers'>
Text-based Markers: the 'list-style-type' property</h3>
<pre class="propdef">
Name: list-style-type
Value: <<counter-style>> | <<string>> | none
Initial: disc
Applies to: <a>list items</a>
Inherited: yes
Percentages: n/a
Computed value: specified value
Animation type: discrete
</pre>
Specifies the <dfn>marker string</dfn>,
which is used to fill the <a>list item</a>’s <a>marker</a>
when its 'content' value is ''content/normal''
and there is no [=marker image=].
The values are as follows:
<dl dfn-type="value" dfn-for="list-style-type">
<dt><dfn><<counter-style>></dfn>
<dd>
Specifies the element’s <a>marker string</a> as
the value of the ''counter()/list-item'' counter
represented using the specified <<counter-style>>.
Specifically,
the <a>marker string</a> is
the result of <a lt="generate a counter representation">generating a counter representation</a>
of the ''counter()/list-item'' counter value
using the specified <<counter-style>>,
prefixed by the '@counter-style/prefix' of the <<counter-style>>,
and followed by the '@counter-style/suffix' of the <<counter-style>>.
If the specified <<counter-style>> does not exist,
''decimal'' is assumed.
<dt><dfn><<string>></dfn>
<dd>
The element’s <a>marker string</a> is the specified <<string>>.
<dt><dfn>none</dfn>
<dd>
The element has no <a>marker string</a>.
</dl>
<div class='example'>
The following examples illustrate how to set markers to various values:
<pre highlight=css>
ul { list-style-type: "★"; }
/* Sets the marker to a "star" character */
p.note {
display: list-item;
list-style-type: "Note: ";
list-style-position: inside;
}
/* Gives note paragraphs a marker consisting of the string "Note: " */
ol { list-style-type: upper-roman; }
/* Sets all ordered lists to use the upper-roman counter-style
(defined in the Counter Styles specification [[CSS-COUNTER-STYLES]]) */
ul { list-style-type: symbols(cyclic '○' '●'); }
/* Sets all unordered list items to alternate between empty and
filled circles for their markers. */
ul { list-style-type: none; }
/* Suppresses the marker entirely, unless list-style-image is specified
with a valid image. */
</pre>
</div>
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<h3 id='list-style-position-property'>
Positioning Markers: The 'list-style-position' property</h3>
<pre class="propdef">
Name: list-style-position
Value: inside | outside
Initial: outside
Applies to: <a>list items</a>
Inherited: yes
Percentages: n/a
Computed value: keyword, but see prose
Animation type: discrete
</pre>
This property dictates whether the ''::marker'' is rendered inline,
or positioned just outside of the <a>list item</a>.
The values are as follows:
<dl dfn-type=value dfn-for=list-style-position>
<dt><dfn>inside</dfn>
<dd>
No special effect.
(The ''::marker'' is an inline element at the start of the <a>list item's</a> contents.)
<dt><dfn id='list-style-position-outside'>outside</dfn>
<dd>
If the <a>list item</a> is a <a>block container</a>:
the marker box is outside the principal block box;
however, the position of the list-item marker adjacent to floats is undefined.
CSS does not specify the precise location of the marker box
or its position in the painting order,
but does require that it be placed on the <a>inline-start</a> side of the box,
using the <a>writing mode</a> of the box indicated by 'marker-side'.
The marker box is fixed with respect to the principal block box's border
and does not scroll with the principal box's content.
A UA may hide the marker if the element's 'overflow' is other than ''overflow/visible''.
(This allowance may change in the future.)
The size or contents of the marker box may affect
the height of the principal block box
and/or the height of its first line box,
and in some cases may cause the creation of a new line box;
this interaction is also not defined.
Issue: This is handwavey nonsense from CSS2, and needs a real definition.
If the <a>list item</a> is an <a>inline box</a>:
this value is equivalent to ''inside''.
Issue: Alternatively, ''outside'' could lay out the marker as a previous sibling of the principal inline box.
</dl>
<div class=example>
For example:
<pre>
<style>
ul.compact { list-style: inside; }
ul { list-style: outside; }
</style>
<ul class=compact>
<li>first "inside" list item comes first</li>
<li>second "inside" list item comes first</li>
</ul>
<hr>
<ul>
<li>first "outside" list item comes first</li>
<li>second "outside" list item comes first</li>
</ul>
</pre>
The above example may be formatted as:
<pre>
* first "inside" list
item comes first
* second "inside" list
item comes second
========================
* first "outside" list
item comes first
* second "outside" list
item comes second</pre>
</div>
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<h3 id='list-style-property'>
Styling Markers: the 'list-style' shorthand property</h3>
<pre class='propdef shorthand'>
Name: list-style
Value: <<'list-style-position'>> || <<'list-style-image'>> || <<'list-style-type'>>
Applies to: <a>list items</a>
</pre>
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.
<div class="example">
For example:
<pre highlight=css>
ul { list-style: upper-roman inside } /* Any UL */
ul ul { list-style: circle outside } /* Any UL child of a UL */
</pre>
</div>
Using a value of <css>none</css> in the shorthand is potentially ambiguous,
as <css>none</css> is a valid value for both 'list-style-image' and 'list-style-type'.
To resolve this ambiguity,
a value of <css>none</css> in the shorthand must be applied to whichever of the two properties aren't otherwise set by the shorthand.
<div class='example'>
<pre highlight=css>
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>
</div>
<div class=example highlight=css>
Although authors may specify 'list-style' information directly on list item elements
(e.g., <{li}> in HTML),
they should do so with care.
Consider the following rules:
<pre>
ol.alpha li { list-style: lower-alpha; }
ul li { list-style: disc; }
</pre>
The above won't work as expected.
If you nest a <{ul}> into an <a element lt="ol">ol class=alpha</a>,
the first rule's specificity will make the <{ul}>’s list items use the lower-alpha style.
<pre>
ol.alpha > li { list-style: lower-alpha; }
ul > li { list-style: disc; }
</pre>
These work as intended.
<pre>
ol.alpha { list-style: lower-alpha; }
ul { list-style: disc; }
</pre>
These are even better,
since inheritance will transfer the 'list-style' value to the list items.
</div>
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<h3 id='marker-side'>
The 'marker-side' property</h3>
<pre class='propdef'>
Name: marker-side
Value: match-self | match-parent
Initial: match-self
Applies to: <a>list items</a>
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Animation type: discrete
</pre>
The 'marker-side' property specifies
whether the ''::marker'' is positioned
based on the directionality of the list item itself (i.e. its [=originating element=])
or the directionality of the list container (i.e. the [=originating element=]’s parent).
In the first case, the position of the marker can vary across items in the same list,
based on the directionality assigned to each list item individually;
in the second case they will all align on the same side,
as determined by the directionality assigned to the list as a whole.
<dl dfn-type=value dfn-for=marker-side>
<dt><dfn>match-self</dfn>
<dd>
The ''::marker'' pseudo-element is positioned
using the directionality of the ''::marker''’s [=originating element=].
<dt><dfn>match-parent</dfn>
<dd>
The ''::marker'' pseudo-element is positioned
using the directionality of the ''::marker''’s [=originating element’s=] parent element.
</dl>
<div class='example'>
By default, elements or ''::marker'' pseudo-elements
position themselves according to their list item's directionality.
However, if the list item is grouped with several other list items which may have different directionality
(for example, multiple <li>s with different "dir" attributes in an <ol> in HTML),
it is sometimes more useful to have all the markers line up on one side,
so the author can specify a single "gutter" on that side
and be assured that all the markers will lie in that gutter and be visible.
Both of the following example renderings are generated from the following HTML,
with the only difference being the value of 'marker-side' on the list:
<pre highlight=html>
<ul>
<li>english one
<li dir=rtl>OWT WERBEH
<li>english three
<li dir=rtl>RUOF WERBEH
</ul>
</pre>
<table class=data style="width: 35em;">
<thead>
<tr>
<th>''match-self''
<th>''match-parent''
<tbody>
<tr>
<td style="border-right: thin solid">
<pre>
* english one
OWT WERBEH *
* english three
RUOF WERBEH *</pre>
<td>
<pre>
* english one
* OWT WERBEH
* english three
* RUOF WERBEH</pre>
</table>
</div>
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<h2 id='auto-numbering'>
Automatic Numbering With Counters</h2>
A <dfn export>counter</dfn> is a special numeric tracker
used, among other things, to automatically number list items in CSS.
Every element has a collection of zero or more counters,
which are inherited through the document tree in a way similar to inherited property values.
Counters have a name and creator element,
which identify the counter,
and an integer value per element.
They are created and manipulated with
the <dfn export>counter properties</dfn> 'counter-increment', 'counter-set' and 'counter-reset',
and used with the ''counter()'' and ''counters()'' [=functional notations=].
Counters are referred to in CSS syntax
using the <dfn><<counter-name>></dfn> type,
which represents their name as a <<custom-ident>>.
A <<counter-name>> name cannot match the keyword ''counter-reset/none'';
such an identifier is [=invalid=] as a <<counter-name>>.
Resolving [=counter=] values on a given element is a multi-step process:
1. Existing counters are [=inherit counters|inherited=] from previous elements.
2. New counters are [=instantiated=] ('counter-reset').
3. Counter values are incremented ('counter-increment').
4. Counter values are explicitly set ('counter-set').
5. Counter values are used (''counter()''/''counters()'').
UAs may have implementation-specific limits
on the maximum or minimum value of a counter.
If a counter reset, set, or increment
would push the value outside of that range,
the value must be clamped to that range.
<h3 id='counter-reset'>
Creating Counters: the 'counter-reset' property</h3>
<pre class="propdef">
Name: counter-reset
Value: [ <<counter-name>> <<integer>>? ]+ | none
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: the keyword ''counter-reset/none'' or a list, each item an identifier paired with an integer
Animation type: by computed value type
</pre>
<p class=all-media>User Agents are expected to support this property on all media, including non-visual ones.
The 'counter-reset' property [=instantiates=] new [=counters=] on an element
and sets them to the specified integer values.
Its values are defined as follows:
<dl dfn-type=value dfn-for=counter-reset>
<dt><dfn>none</dfn>
<dd>
This element does not create any new counters.
<dt><dfn><<counter-name>> <<integer>>?</dfn>
<dd>
[=Instantiates=] a counter of the given <<counter-name>> <dfn>counter name</dfn>
with a starting value of the given <<integer>>,
defaulting to ''0''.
</dl>
<div class='example' highlight=css>
Note that counter properties follow the [=cascading=] rules as normal.
Thus, due to cascading, the following style sheet:
<pre>
h1 { counter-reset: section -1 }
h1 { counter-reset: imagenum 99 }
</pre>
will only reset ''imagenum''.
To reset both counters,
they have to be specified together:
<pre>H1 { counter-reset: section -1 imagenum 99 }</pre>
The same principles apply to the 'counter-set' and 'counter-increment' properties.
See [[css-cascade-4]].
</div>
If multiple instances of the same <<counter-name>> occur in the property value,
only the last one is honored.
<h3 id='increment-set'>
Manipulating Counter Values: the 'counter-increment' and 'counter-set' properties</h3>
<pre class='propdef'>
Name: counter-increment
Value: [ <<counter-name>> <<integer>>? ]+ | none
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: the keyword ''counter-reset/none'' or a list, each item an identifier paired with an integer
Animation type: by computed value type
</pre>
<p class=all-media>User Agents are expected to support this property on all media, including non-visual ones.
<pre class='propdef'>
Name: counter-set
Value: [ <<counter-name>> <<integer>>? ]+ | none
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: the keyword ''counter-reset/none'' or a list, each item an identifier paired with an integer
Animation type: by computed value type
</pre>
<p class=all-media>User Agents are expected to support this property on all media, including non-visual ones.
The 'counter-increment' and 'counter-set' properties
manipulate the value of existing [=counters=].
They only [=instantiate=] new counters
if there is no counter of the given name on the element yet.
Their values are defined as follows:
<dl dfn-type=value dfn-for="counter-set counter-increment">
<dt><dfn>none</dfn>
<dd>
This element does not alter the value of any counters.
<dt><dfn><<counter-name>> <<integer>>?</dfn>
<dd>
Sets (for 'counter-set') or increments (for 'counter-increment')
the value of the named counter on the element
to/by the specified <<integer>>.
If the <<integer>> is omitted,
it defaults to ''1'' (for 'counter-increment')
or ''0'' (for 'counter-set').
If there is not currently a counter of the given name on the element,
the element [=instantiates=] a new counter of the given name
with a starting value of ''0''
before setting or incrementing its value.
</dl>
<div class="example">
<p>This example shows a way to number chapters and sections with "Chapter 1", "1.1", "1.2", etc.
<pre highlight=css>
h1::before {
content: "Chapter " counter(chapter) ". ";
counter-increment: chapter; /* Add 1 to chapter */
counter-reset: section; /* Set section to 0 */
}
h2::before {
content: counter(chapter) "." counter(section) " ";
counter-increment: section;
}
</pre>
</div>
If multiple instances of the same <<counter-name>> occur in the property value,
they are all processed, in order.
Thus increments will compound,
but only the last set value will take effect.
<h3 id='nested-counters'>
Nested Counters and Scope</h3>
Counters are “self-nesting”;
[=instantiating=] a new counter on an element
which inherited an identically-named [=counter=]
from its parent
creates a new counter of the same name,
nested inside the existing counter.
This is important for situations like lists in HTML,
where lists can be nested inside lists to arbitrary depth:
it would be impossible to define uniquely named counters for each level.
The ''counter()'' function only retrieves the [=innermost=] counter of a given name on the element,
whereas the ''counters()'' function uses all counters of a given name that contain the element.
The <dfn lt="counter scope" local-lt="scope">scope</dfn> of a [=counter=] therefore
starts at the first element in the document that [=instantiates=] that counter
and includes the element’s descendants and its following siblings with their descendants.
However, it does not include any elements in the scope of a counter with the same name
created by a 'counter-reset' on a later sibling of the element,
allowing such explicit counter instantiations
to obscure those earlier siblings.
See [[#creating-counters]] for the exact rules governing the scope of counters and their values.
<div class="example" id='counter-nesting-example'>
The following code numbers nested list items.
The result is very similar to that of setting ''display:list-item'' and ''list-style: inside'' on the LI element:
<pre highlight=css>
ol { counter-reset: item }
li { display: block }
li::before { content: counter(item) ". "; counter-increment: item }
</pre>
In this example,
an <a element>ol</a> will create a counter,
and all children of the <a element>ol</a> will refer to that counter.
If we denote the <var>n</var><sup>th</sup> instance of the ''item'' counter by item<sub><var>n</var></sub>,
then the following HTML fragment will use the indicated counters.
<div class='ol'>
<div class='ol'><code><ol></code> item<sub>0</sub> is created, set to 0
<div class='li'><code><li></code> item<sub>0</sub> is incremented to 1</div>
<div class='li'><code><li></code> item<sub>0</sub> is incremented to 2
<div class='ol'><code><ol></code> item<sub>1</sub> is created, set to 0, nested in item<sub>0</sub>
<div class='li'><code><li></code> item<sub>1</sub> is incremented to 1</div>
<div class='li'><code><li></code> item<sub>1</sub> is incremented to 2</div>
<div class='li'><code><li></code> item<sub>1</sub> is incremented to 3
<div class='ol'><code><ol></code> item<sub>2</sub> is created, set to 0, nested in item<sub>1</sub>
<div class='li'><code><li></code> item<sub>2</sub> is incremented to 1</div>
<code></ol></code>
</div>
</div>
<div class='li'><code><li></code> item<sub>1</sub> is incremented to 4
<div class='ol'><code><ol></code> item<sub>3</sub> is created, set to 0, nested in item<sub>1</sub>
<div class='li'><code><li></code> item<sub>3</sub> is incremented to 1</div>
<code></ol></code>
</div>
</div>
<div class='li'><code><li></code> item<sub>1</sub> is incremented to 5</div>
<code></ol></code>
</div>
</div>
<div class='li'><code><li></code> item<sub>0</sub> is incremented to 3</div>
<div class='li'><code><li></code> item<sub>0</sub> is incremented to 4</div>
<code></ol></code>
</div>
<div class='ol'><code><ol></code> item<sub>4</sub> is created, set to 0
<div class='li'><code><li></code> item<sub>4</sub> is incremented to 1</div>
<div class='li'><code><li></code> item<sub>4</sub> is incremented to 2</div>
<code></ol></code>
</div>
</div>
<style>
#counter-nesting-example .ol { background: rgba(0,0,0,.1); margin: .5em 0; padding: .2em .5em; }
#counter-nesting-example .li > .ol { margin: 0 0 0 1em; }
#counter-nesting-example .li { list-style: none; margin-left: 1em;}
</style>
</div>
<h3 id="creating-counters">
Creating and Inheriting Counters</h3>
Each element or pseudo-element in a document has
a (possibly empty) set of [=counters=] in the [=scope=] of that element,
either through inheritance from another element
or through instantiation on the element directly.
These counters are represented as a <dfn>CSS counters set</dfn>,
which is a [=/set=]
whose values are each a [=tuple=] of:
a [=string=] (representing a counter’s name),
an element (representing the counter’s originating element),
and an integer (representing the counter’s value).
The latest [=counter=] of a given name in that set
represents the <dfn>innermost</dfn> counter of that name.
<h4 id="inheriting-counters">
Inheriting Counters</h4>
<div algorithm>
An element [=inherit counters|inherits=] its initial set of counters
from its preceding <em>sibling</em>,
or from its parent if it has none.
It then takes the values for those counters
from the values of the matching counters
on its preceding <em>element in [=tree order=]</em>
(which might be its parent,
its preceding sibling,
or a descendant of its previous sibling).
To <dfn>inherit counters</dfn>
into an |element|:
1. Let |element counters| be an initially empty [=CSS counters set=]
representing |element|’s own [=CSS counters set=].
2. If |element| is the [=tree/root=] of its document tree,
return.
(The element has an initially-empty counter map
and inherits nothing.)
3. Let |counter source| be the [=CSS counters set=]
of |element|’s preceding sibling,
if it has one,
or else of |element|’s parent
if it does not.
4. Let |value source| be the [=CSS counters set=]
of the element immediately preceding |element| in [=tree order=].
5. [=map/For each=] (|name|, |originating element|, |value|) of |value source|:
1. If |counter source| also [=set/contains=] a [=counter=]
with the same |name| and |originating element|,
then [=set/append=] a copy of |value source|’s [=counter=]
(|name|, |originating element|, |value|)
to |element counters|.
</div>
<div class='example' id='counter-inheritance-example'>
Take the following code as an example:
<pre>
<ul style='counter-reset: example 0;'>
<b class='foo'><li id='foo' style='counter-increment: example;'></b>
foo
<b class='bar'><div id='bar' style='counter-increment: example;'>bar</div></b>
</li>
<b class='baz'><li id='baz'></b>
baz
</li>
</ul>
</pre>
Recall that [=tree order=] turns a document tree into an ordered list,
where an element comes before its children,
and its children come before its next sibling.
In other words,
for a language like HTML,
it's the order in which the parser encounters start tags as it reads the document.
In here, the <{ul}> element establishes a new counter named ''example'',
and sets its value to ''0''.
The <b class='foo'>#foo</b> element, being the first child of the <{ul}>,
inherits this counter.
Its parent is also its immediately preceding element in [=tree order=],
so it inherits the value ''0'' with it,
and then immediately increments the value to ''1''.
The same happens with the <b class='bar'>#bar</b> element.
It inherits the ''example'' counter from <b class='foo'>#foo</b>,
and inherits the value ''1'' from it as well
and increments it to ''2''.
However, the <b class='baz'>#baz</b> element is a bit different.
It inherits the ''example'' counter from the <b class='foo'>#foo</b> element,
its previous sibling.
However, rather than inheriting the value ''1'' from <b class='foo'>#foo</b> along with the counter,
in inherits the value ''2'' from <b class='bar'>#bar</b>, the previous element in [=tree order=].
This behavior allows a single counter to be used throughout a document,
continuously incrementing,
without the author having to worry about the nested structure of their document.
</div>
<style>
#counter-inheritance-example b.foo { color: red; }
#counter-inheritance-example b.bar { color: green; }
#counter-inheritance-example b.baz { color: blue; }
</style>
Note: Counter inheritance, like regular CSS [=inheritance=],
operates on the “flattened element tree”
in the context of the [[DOM]].
<h4 id="instantiating-counters">