Selectors Level 4

Abstract

Selectors are patterns that match against elements in a tree, and as such form one of several technologies that can be used to select nodes in an XML document. Selectors have been optimized for use with HTML and XML, and are designed to be usable in performance-critical code. They are a core component of CSS (Cascading Style Sheets), which uses Selectors to bind style properties to elements in the document.

Selectors Level 4 describes the selectors that already exist in [[!CSS3SELECT]], and further introduces new selectors for CSS and other languages that may need them.

Status of this document

Table of contents

Introduction

Background

This section is not normative.

Selectors define the following function:

expression ∗ element → boolean

That is, given an element and a selector, this specification defines whether that element matches the selector.

These expressions can also be used, for instance, to select a set of elements, or a single element from a set of elements, by evaluating the expression across all the elements in a subtree. STTS (Simple Tree Transformation Sheets), a language for transforming XML trees, uses this mechanism. [[STTS3]]

Selectors Levels 1, 2, and 3 are defined as the subsets of selector functionality defined in the CSS1, CSS2.1, and Selectors Level 3 specifications, respectively.

Module Interactions

This module replaces the definitions for, and extends the set of selectors (aside from pseudo-element selectors) defined for CSS in [[CSS3SELECT]] and [[CSS21]].

Pseudo-element selectors, which define abstract elements in a rendering tree, are not part of this specification: their generic syntax is described here, but they are defined in a separate Pseudo-Element Selectors Level 4 module.

Document Conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [[!RFC2119]]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Selectors Overview

This section is non-normative, as it merely summarizes the following sections.

A Selector represents a structure. This structure can be used as a condition (e.g. in a CSS rule) that determines which elements a selector matches in the document tree, or as a flat description of the HTML or XML fragment corresponding to that structure.

Selectors may range from simple element names to rich contextual representations.

The following table summarizes the Selector syntax:

The meaning of each selector is derived from the table above by prepending "matches" to the contents of each cell in the "Meaning" column.

Selector Syntax

Structure and Terminology

A selector is a chain of one or more compound selectors separated by combinators.

A compound selector is a chain of simple selectors that are not separated by a combinator. It always begins with a type selector or a (possibly implied) universal selector. No other type selector or universal selector is allowed in the sequence.

A simple selector is either a type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class.

Combinators are punctuation that represent a particular kind of relationship between the compound selectors on either side. Combinators in Selectors level 4 include: whitespace, "greater-than sign" (U+003E, >), "plus sign" (U+002B, +) and "tilde" (U+007E, ~). White space may appear between a combinator and the simple selectors around it.

An empty selector, containing no compound selector, is an invalid selector.

Subject of a Selector

The elements of a document tree that are represented by a selector are the subjects of the selector.

By default, the subjects of a selector are the elements represented by the last compound selector in the selector. Thus a selector consisting of a single compound selector represents any element satisfying its requirements. Prepending another compound selector and a combinator to a sequence imposes additional matching constraints, so the subjects of the selector are always a subset of the elements represented by the last compound selector.

The subject of the selector can be explicitly identified by prepending an exclamation mark (!) to one of the compound selectors in a selector. Although the element structure that the selector represents is the same with or without the exclamation mark, indicating the subject in this way can change which compound selector represents the subject in that structure.

For example, the following selector represents a list item LI unique child of an ordered list OL:

OL > LI:only-child

However the following one represents an ordered list OL having a unique child, that child being a LI:

!OL > LI:only-child

The structures represented by these two selectors are the same, but the subjects of the selectors are not.

Characters and case sensitivity

All Selectors syntax is case-insensitive within the ASCII range (i.e. [a-z] and [A-Z] are equivalent), except for parts that are not under the control of Selectors. The case sensitivity of document language element names, attribute names, and attribute values in selectors depends on the document language. For example, in HTML, element names are case-insensitive, but in XML, they are case-sensitive. Case sensitivity of namespace prefixes is defined in [[!CSS3NAMESPACE]].

White space in Selectors consists of the characters SPACE (U+0020), TAB (U+0009), LINE FEED (U+000A), CARRIAGE RETURN (U+000D), and FORM FEED (U+000C) can occur in whitespace. Other space-like characters, such as EM SPACE (U+2003) and IDEOGRAPHIC SPACE (U+3000), are never part of white space.

Characters in Selectors can be escaped with a backslash according to the same escaping rules as CSS. [[!CSS21]].

Namespaces

Certain selectors support namespace prefixes. The mechanism by which namespace prefixes are declared should be specified by the language that uses Selectors. If the language does not specify a namespace prefix declaration mechanism, then no prefixes are declared. In CSS, namespace prefixes are declared with the @namespace rule. [[!CSS3NAMESPACE]]

Invalid Selectors and Error Handling

Invalidity is caused by a parsing error, e.g. an unrecognized token or a token which is not allowed at the current parsing point.

User agents must observe the rules for handling parsing errors:

• a simple selector containing an undeclared namespace prefix is invalid
• a selector containing an invalid simple selector, an invalid combinator or an invalid token is invalid.
• a group of selectors containing an invalid selector is invalid.

It's been requested that the last rule be dropped in favor of Media Queries-style error-handling.

Logical Combinations

Selector Groups

A comma-separated list of selectors represents the union of all elements selected by each of the individual selectors in the list. (A comma is U+002C.) For example, in CSS when several selectors share the same declarations, they may be grouped into a comma-separated list. White space may appear before and/or after the comma.

h1 { font-family: sans-serif }
  h2 { font-family: sans-serif }
  h3 { font-family: sans-serif }

h1, h2, h3 { font-family: sans-serif }

Warning: the equivalence is true in this example because all the selectors are valid selectors. If just one of these selectors were invalid, the entire group of selectors would be invalid. This would invalidate the rule for all three heading elements, whereas in the former case only one of the three individual heading rules would be invalidated.

h1 { font-family: sans-serif }
  h2..foo { font-family: sans-serif }
  h3 { font-family: sans-serif }

h1, h2..foo, h3 { font-family: sans-serif }

The Matches-Any Pseudo-class: '':matches()''

The matches pseudo-class, :matches(X), is a functional notation taking a selector group as an argument. It represents an element that is represented by its argument.

In Selectors Level 4, only compound selectors are allowed within :matches(): combinators are not allowed. Additionally, :matches() may not be nested within itself or within :not(): :matches(:matches(...)) and :not(:matches(...)) are invalid.

Pseudo-elements cannot be represented by the matches pseudo-class; they are not valid within :matches().

Default namespace declarations do not affect the subject of any selector within a matches-any pseudo-class unless the argument is an explicit universal selector or a type selector.

*|*:matches(:hover, :focus)

*|*:matches(*:hover, *:focus)

The Negation Pseudo-class: '':not()''

The negation pseudo-class, :not(X), is a functional notation taking a selector group as an argument. It represents an element that is not represented by its argument.

In Selectors Level 4, only compound selectors are allowed within :matches(): combinators are not allowed. Additionally, negations may not be nested within itself or within :matches(): :not(:not(...)) and :matches(:not(...)) are invalid.

Pseudo-elements cannot be represented by the negation pseudo-class; they are not valid within :not().

button:not([DISABLED])

*:not(FOO)

html|*:not(:link):not(:visited)

Default namespace declarations do not affect the subject of any selector within a negation pseudo-class unless the argument is an explicit universal selector or a type selector. (See :matches() for examples.

Note: the :not() pseudo allows useless selectors to be written. For instance :not(*|*), which represents no element at all, or foo:not(bar), which is equivalent to foo but with a higher specificity.

Elemental selectors

Type selector

A type selector is the name of a document language element type written using the syntax of CSS qualified names [[!CSS3NAMESPACE]]. A type selector represents an instance of the element type in the document tree.

h1

Type selectors and namespaces

Type selectors allow an optional namespace component: a namespace prefix that has been previously declared may be prepended to the element name separated by the namespace separator "vertical bar" (U+007C, |). (See, e.g., [[XML-NAMES]] for the use of namespaces in XML.)

The namespace component may be left empty (no prefix before the namespace separator) to indicate that the selector is only to represent elements with no namespace.

An asterisk may be used for the namespace prefix, indicating that the selector represents elements in any namespace (including elements with no namespace).

Element type selectors that have no namespace component (no namespace separator) represent elements without regard to the element's namespace (equivalent to "*|") unless a default namespace has been declared for namespaced selectors (e.g. in CSS, in the style sheet). If a default namespace has been declared, such selectors will represent only elements in the default namespace.

A type selector containing a namespace prefix that has not been previously declared for namespaced selectors is an invalid selector.

In a namespace-aware client, the name part of element type selectors (the part after the namespace separator, if it is present) will only match against the local part of the element's qualified name.

In summary:

ns|E

elements with name E in namespace ns

*|E

elements with name E in any namespace, including those without a namespace

|E

elements with name E without a namespace

E

if no default namespace has been declared for selectors, this is equivalent to *|E. Otherwise it is equivalent to ns|E where ns is the default namespace.

@namespace foo url(http://www.example.com);
   foo|h1 { color: blue }  /* first rule */
   foo|* { color: yellow } /* second rule */
   |h1 { color: red }      /* ...*/
   *|h1 { color: green }
   h1 { color: green }

Universal selector

The universal selector, written as a CSS qualified name [[!CSS3NAMESPACE]] with an asterisk (* U+002A) as the local name, represents the qualified name of any element type. It represents any single element in the document tree in any namespace (including those without a namespace) if no default namespace has been specified for selectors. If a default namespace has been specified, see Universal selector and Namespaces below.

If a universal selector represented by * (i.e. without a namespace prefix) is not the only component of a compound selector or is immediately followed by a pseudo-element, then the * may be omitted and the universal selector's presence implied.

Note: it is recommended that the * not be omitted, because it decreases the potential confusion between, for example, div :first-child and div:first-child. Here, div *:first-child is more readable.

Universal selector and namespaces

The universal selector allows an optional namespace component. It is used as follows:

ns|*

all elements in namespace ns

*|*

all elements

|*

all elements without a namespace

*

if no default namespace has been specified, this is equivalent to *|*. Otherwise it is equivalent to ns|* where ns is the default namespace.

A universal selector containing a namespace prefix that has not been previously declared is an invalid selector.

Attribute selectors

Selectors allow the representation of an element's attributes. When a selector is used as an expression to match against an element, attribute selectors must be considered to match an element if that element has an attribute that matches the attribute represented by the attribute selector.

Attribute presence and value selectors

CSS2 introduced four attribute selectors:

[att]

Represents an element with the att attribute, whatever the value of the attribute.

[att=val]

Represents an element with the att attribute whose value is exactly "val".

[att~=val]

Represents an element with the att attribute whose value is a whitespace-separated list of words, one of which is exactly "val". If "val" contains whitespace, it will never represent anything (since the words are separated by spaces). Also if "val" is the empty string, it will never represent anything.

[att|=val]

Represents an element with the att attribute, its value either being exactly "val" or beginning with "val" immediately followed by "-" (U+002D). This is primarily intended to allow language subcode matches (e.g., the hreflang attribute on the a element in HTML) as described in BCP 47 ([[BCP47]]) or its successor. For lang (or xml:lang) language subcode matching, please see the :lang pseudo-class.

Attribute values must be CSS identifiers or strings. [[!CSS21]] The case-sensitivity of attribute names and values in selectors depends on the document language.

h1[title]

span[class="example"]

span[hello="Cleveland"][goodbye="Columbus"]

a[rel~="copyright"] { ... }
  a[href="http://www.w3.org/"] { ... }

a[hreflang=fr]

a[hreflang|="en"]

DIALOGUE[character=romeo]
  DIALOGUE[character=juliet]

Substring matching attribute selectors

Three additional attribute selectors are provided for matching substrings in the value of an attribute:

[att^=val]

Represents an element with the att attribute whose value begins with the prefix "val". If "val" is the empty string then the selector does not represent anything.

[att$=val]

Represents an element with the att attribute whose value ends with the suffix "val". If "val" is the empty string then the selector does not represent anything.

[att*=val]

Represents an element with the att attribute whose value contains at least one instance of the substring "val". If "val" is the empty string then the selector does not represent anything.

Attribute values must be CSS identifiers or strings. [[!CSS21]] The case-sensitivity of attribute names in selectors depends on the document language.

object[type^="image/"]

a[href$=".html"]

p[title*="hello"]

Attribute selectors and namespaces

The attribute name in an attribute selector is given as a CSS qualified name: a namespace prefix that has been previously declared may be prepended to the attribute name separated by the namespace separator "vertical bar" (|). In keeping with the Namespaces in the XML recommendation, default namespaces do not apply to attributes, therefore attribute selectors without a namespace component apply only to attributes that have no namespace (equivalent to "|attr"; these attributes are said to be in the "per-element-type namespace partition"). An asterisk may be used for the namespace prefix indicating that the selector is to match all attribute names without regard to the attribute's namespace.

An attribute selector with an attribute name containing a namespace prefix that has not been previously declared is an invalid selector.

@namespace foo "http://www.example.com";
  [foo|att=val] { color: blue }
  [*|att] { color: yellow }
  [|att] { color: green }
  [att] { color: green }

Default attribute values in DTDs

Attribute selectors represent attribute values in the document tree. How that document tree is constructed is outside the scope of Selectors. In some document formats default attribute values can be defined in a DTD or elsewhere, but these can only be selected by attribute selectors if they appear in the document tree. Selectors should be designed so that they work whether or not the default values are included in the document tree.

For example, a XML UA may, but is not required to read an "external subset" of the DTD but is required to look for default attribute values in the document's "internal subset." (See, e.g., [[XML10]] for definitions of these subsets.) Depending on the UA, a default attribute value defined in the external subset of the DTD might or might not appear in the document tree.

A UA that recognizes an XML namespace may, but is not required to use its knowledge of that namespace to treat default attribute values as if they were present in the document. (For example, an XHTML UA is not required to use its built-in knowledge of the XHTML DTD. See, e.g., [[XML-NAMES]] for details on namespaces in XML 1.0.)

Note: Typically, implementations choose to ignore external subsets. This corresponds to the behaviour of non-validating processors as defined by the XML specification.

<!ATTLIST EXAMPLE radix (decimal,octal) "decimal">

EXAMPLE[radix=decimal] { /*... default property settings ...*/ }
  EXAMPLE[radix=octal]   { /*... other settings...*/ }

EXAMPLE                { /*... default property settings ...*/ }
  EXAMPLE[radix=octal]   { /*... other settings...*/ }

Class selectors

Working with HTML, authors may use the "period" notation (also known as "full stop", U+002E, .) as an alternative to the ~= notation when representing the class attribute. Thus, for HTML, div.value and div[class~=value] have the same meaning. The attribute value must immediately follow the full stop (.).

UAs may apply selectors using the period (.) notation in XML documents if the UA has namespace-specific knowledge that allows it to determine which attribute is the "class" attribute for the respective namespace. One such example of namespace-specific knowledge is the prose in the specification for a particular namespace (e.g. SVG 1.0 [[SVG11]] describes the SVG class attribute and how a UA should interpret it, and similarly MathML 1.01 [[MATHML]] describes the MathML class attribute.)

*.pastoral { color: green }  /* all elements with class~=pastoral */

.pastoral { color: green }  /* all elements with class~=pastoral */

H1.pastoral { color: green }  /* H1 elements with class~=pastoral */

<H1>Not green</H1>
  <H1 class="pastoral">Very green</H1>

p.pastoral.marine { color: green }

Note: Because CSS gives considerable power to the "class" attribute, authors could conceivably design their own "document language" based on elements with almost no associated presentation (such as DIV and SPAN in HTML) and assigning style information through the "class" attribute. Authors should avoid this practice since the structural elements of a document language often have recognized and accepted meanings and author-defined classes may not.

Note: If an element has multiple class attributes, their values must be concatenated with spaces between the values before searching for the class. As of this time the working group is not aware of any manner in which this situation can be reached, however, so this behavior is explicitly non-normative in this specification.

ID selectors

Document languages may contain attributes that are declared to be of type ID. What makes attributes of type ID special is that no two such attributes can have the same value in a conformant document, regardless of the type of the elements that carry them; whatever the document language, an ID typed attribute can be used to uniquely identify its element. In HTML all ID attributes are named "id"; XML applications may name ID attributes differently, but the same restriction applies.

An ID-typed attribute of a document language allows authors to assign an identifier to one element instance in the document tree. An ID selector contains a "number sign" (U+0023, #) immediately followed by the ID value, which must be an CSS identifiers. An ID selector represents an element instance that has an identifier that matches the identifier in the ID selector.

Selectors does not specify how a UA knows the ID-typed attribute of an element. The UA may, e.g., read a document's DTD, have the information hard-coded or ask the user.

h1#chapter1

#chapter1

*#z98y

Note: In XML 1.0 [[XML10]], the information about which attribute contains an element's IDs is contained in a DTD or a schema. When parsing XML, UAs do not always read the DTD, and thus may not know what the ID of an element is (though a UA may have namespace-specific knowledge that allows it to determine which attribute is the ID attribute for that namespace). If a style sheet author knows or suspects that a UA may not know what the ID of an element is, he should use normal attribute selectors instead: [name=p371] instead of #p371.

If an element has multiple ID attributes, all of them must be treated as IDs for that element for the purposes of the ID selector. Such a situation could be reached using mixtures of xml:id, DOM3 Core, XML DTDs, and namespace-specific knowledge.

Pseudo-classes

The pseudo-class concept is introduced to permit selection based on information that lies outside of the document tree or that cannot be expressed using the other simple selectors.

A pseudo-class always consists of a "colon" (:) followed by the name of the pseudo-class and optionally by a value between parentheses.

Pseudo-classes are allowed in all compound selectors contained in a selector. Pseudo-classes are allowed anywhere in compound selector after the leading type selector or (possibly omitted) universal selector. Pseudo-class names are case-insensitive. Some pseudo-classes are mutually exclusive, while others can be applied simultaneously to the same element. Pseudo-classes may be dynamic, in the sense that an element may acquire or lose a pseudo-class while a user interacts with the document.

Dynamic pseudo-classes

Dynamic pseudo-classes classify elements on characteristics other than their name, attributes, or content, in principle characteristics that cannot be deduced from the document tree.

Dynamic pseudo-classes do not appear in the document source or document tree.

The link pseudo-classes: :link and :visited

User agents commonly display unvisited links differently from previously visited ones. Selectors provides the pseudo-classes :link and :visited to distinguish them:

• The :link pseudo-class applies to links that have not yet been visited.
• The :visited pseudo-class applies once the link has been visited by the user.

After some amount of time, user agents may choose to return a visited link to the (unvisited) ':link' state.

The two states are mutually exclusive.

a.external:visited

Note: It is possible for style sheet authors to abuse the :link and :visited pseudo-classes to determine which sites a user has visited without the user's consent.

UAs may therefore treat all links as unvisited links, or implement other measures to preserve the user's privacy while rendering visited and unvisited links differently.

The user action pseudo-classes :hover, :active, and :focus

Interactive user agents sometimes change the rendering in response to user actions. Selectors provides three pseudo-classes for the selection of an element the user is acting on.

• The :hover pseudo-class applies while the user designates an element with a pointing device, but does not necessarily activate it. For example, a visual user agent could apply this pseudo-class when the cursor (mouse pointer) hovers over a box generated by the element. User agents not that do not support interactive media do not have to support this pseudo-class. Some conforming user agents that support interactive media may not be able to support this pseudo-class (e.g., a pen device that does not detect hovering).
• The :active pseudo-class applies while an element is being activated by the user. For example, between the times the user presses the mouse button and releases it. On systems with more than one mouse button, :active applies only to the primary or primary activation button (typically the "left" mouse button), and any aliases thereof.
• The :focus pseudo-class applies while an element has the focus (accepts keyboard or mouse events, or other forms of input).

There may be document language or implementation specific limits on which elements can become :active or acquire :focus.

These pseudo-classes are not mutually exclusive. An element may match several pseudo-classes at the same time.

Selectors doesn't define if the parent of an element that is ':active' or ':hover' is also in that state.

Note: If the ':hover' state applies to an element because its child is designated by a pointing device, then it's possible for ':hover' to apply to an element that is not underneath the pointing device.

a:link    /* unvisited links */
  a:visited /* visited links */
  a:hover   /* user hovers */
  a:active  /* active links */

a:focus
  a:focus:hover

Note: An element can be both ':visited' and ':active' (or ':link' and ':active').

The target pseudo-class :target

Some URIs refer to a location within a resource. This kind of URI ends with a "number sign" (#) followed by an anchor identifier (called the fragment identifier).

URIs with fragment identifiers link to a certain element within the document, known as the target element. For instance, here is a URI pointing to an anchor named section_2 in an HTML document:

http://example.com/html/top.html#section_2

A target element can be represented by the :target pseudo-class. If the document's URI has no fragment identifier, then the document has no target element.

p.note:target

*:target { color : red }
  *:target::before { content : url(target.png) }

The language pseudo-class :lang

If the document language specifies how the human language of an element is determined, it is possible to write selectors that represent an element based on its language. For example, in HTML [[HTML401]], the language is determined by a combination of the lang attribute and possibly information from the meta elements or the protocol (such as HTTP headers). XML uses an attribute called xml:lang, and there may be other document language-specific methods for determining the language.

The pseudo-class :lang(C) represents an element that is in language C. Whether an element is represented by a :lang() selector is based solely on the element's language value (normalized to BCP 47 syntax if necessary) being equal to the identifier C, or beginning with the identifier C immediately followed by "-" (U+002D). The matching of C against the element's language value is performed case-insensitively within the ASCII range. The identifier C does not have to be a valid language name.

C must be a valid CSS identifier [[!CSS21]] and must not be empty. (Otherwise, the selector is invalid.)

Note: It is recommended that documents and protocols indicate language using codes from BCP 47 [[BCP47]] or its successor, and by means of "xml:lang" attributes in the case of XML-based documents [[XML10]]. See "FAQ: Two-letter or three-letter language codes."

html:lang(fr-be)
  html:lang(de)
  :lang(fr-be) > q
  :lang(de) > q

The difference between :lang(C) and the '|=' operator is that the '|=' operator only performs a comparison against a given attribute on the element, while the :lang(C) pseudo-class uses the UAs knowledge of the document's semantics to perform the comparison.

<body lang=fr>
    <p>Je suis français.</p>
  </body>

The UI element states pseudo-classes

The :enabled and :disabled pseudo-classes

The :enabled pseudo-class represents user interface elements that are in an enabled state; such elements have a corresponding disabled state.

Conversely, the :disabled pseudo-class represents user interface elements that are in a disabled state; such elements have a corresponding enabled state.

What constitutes an enabled state, a disabled state, and a user interface element is language-dependent. In a typical document most elements will be neither :enabled nor :disabled.

Note: CSS properties that might affect a user’s ability to interact with a given user interface element do not affect whether it matches :enabled or :disabled; e.g., the display and visibility properties have no effect on the enabled/disabled state of an element.

The :checked pseudo-class

Radio and checkbox elements can be toggled by the user. Some menu items are "checked" when the user selects them. When such elements are toggled "on" the :checked pseudo-class applies. While the :checked pseudo-class is dynamic in nature, and can altered by user action, since it can also be based on the presence of semantic attributes in the document, it applies to all media. For example, the :checked pseudo-class initially applies to such elements that have the HTML4 selected and checked attributes as described in Section 17.2.1 of HTML4, but of course the user can toggle "off" such elements in which case the :checked pseudo-class would no longer apply.

The :indeterminate pseudo-class

Radio and checkbox elements can be toggled by the user, but are sometimes in an indeterminate state, neither checked nor unchecked. This can be due to an element attribute, or DOM manipulation.

A future version of this specification may introduce an :indeterminate pseudo-class that applies to such elements. While the :indeterminate pseudo-class is dynamic in nature, and is altered by user action, since it can also be based on the presence of an element attribute, it applies to all media.

Components of a radio-group initialized with no pre-selected choice are an example of :indeterminate state.

Structural pseudo-classes

Selectors introduces the concept of structural pseudo-classes to permit selection based on extra information that lies in the document tree but cannot be represented by other simple selectors or combinators.

Standalone text and other non-element nodes are not counted when calculating the position of an element in the list of children of its parent. When calculating the position of an element in the list of children of its parent, the index numbering starts at 1.

:root pseudo-class

The :root pseudo-class represents an element that is the root of the document. In HTML 4, this is always the HTML element.

:nth-child() pseudo-class

The :nth-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings before it in the document tree, for any positive integer or zero value of n, and has a parent element. For values of a and b greater than zero, this effectively divides the element's children into groups of a elements (the last group taking the remainder), and selecting the bth element of each group. For example, this allows the selectors to address every other row in a table, and could be used to alternate the color of paragraph text in a cycle of four. The a and b values must be integers (positive, negative, or zero). The index of the first child of an element is 1.

In addition to this, :nth-child() can take 'odd' and 'even' as arguments instead. 'odd' has the same signification as 2n+1, and 'even' has the same signification as 2n.

The argument to :nth-child() must match the grammar below, where INTEGER matches the token [0-9]+ and the rest of the tokenization is given by the Lexical scanner in section 10.2:

nth
    : S* [ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]? |
           ['-'|'+']? INTEGER | {O}{D}{D} | {E}{V}{E}{N} ] S*
    ;

tr:nth-child(2n+1) /* represents every odd row of an HTML table */
  tr:nth-child(odd)  /* same */
  tr:nth-child(2n+0) /* represents every even row of an HTML table */
  tr:nth-child(even) /* same */

  /* Alternate paragraph colours in CSS */
  p:nth-child(4n+1) { color: navy; }
  p:nth-child(4n+2) { color: green; }
  p:nth-child(4n+3) { color: maroon; }
  p:nth-child(4n+4) { color: purple; }

When the value b is preceded by a negative sign, the "+" character in the expression must be removed (it is effectively replaced by the "-" character indicating the negative value of b).

:nth-child(10n-1)  /* represents the 9th, 19th, 29th, etc, element */
  :nth-child(10n+9)  /* Same */
  :nth-child(10n+-1) /* Syntactically invalid, and would be ignored */

When a=0, the an part need not be included (unless the b part is already omitted). When an is not included and b is non-negative, the + sign before b (when allowed) may also be omitted. In this case the syntax simplifies to :nth-child(b).

foo:nth-child(0n+5)   /* represents an element foo that is the 5th child
                           of its parent element */
  foo:nth-child(5)      /* same */

When a=1, or a=-1, the 1 may be omitted from the rule.

bar:nth-child(1n+0)   /* represents all bar elements, specificity (0,1,1) */
  bar:nth-child(n+0)    /* same */
  bar:nth-child(n)      /* same */
  bar                   /* same but lower specificity (0,0,1) */

If b=0, then every ath element is picked. In such a case, the +b (or -b) part may be omitted unless the a part is already omitted.

tr:nth-child(2n+0) /* represents every even row of an HTML table */
  tr:nth-child(2n) /* same */

Whitespace is permitted after the "(", before the ")", and on either side of the "+" or "-" that separates the an and b parts when both are present.

  :nth-child( 3n + 1 )
  :nth-child( +3n - 2 )
  :nth-child( -n+ 6)
  :nth-child( +6 )
  

  :nth-child(3 n)
  :nth-child(+ 2n)
  :nth-child(+ 2)
  

If both a and b are equal to zero, the pseudo-class represents no element in the document tree.

The value a can be negative, but only the positive values of an+b, for n≥0, may represent an element in the document tree.

html|tr:nth-child(-n+6)  /* represents the 6 first rows of XHTML tables */

:nth-last-child() pseudo-class

The :nth-last-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings after it in the document tree, for any positive integer or zero value of n, and has a parent element. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values as arguments.

tr:nth-last-child(-n+2)    /* represents the two last rows of an HTML table */

  foo:nth-last-child(odd)    /* represents all odd foo elements in their parent element,
                                counting from the last one */

:nth-of-type() pseudo-class

The :nth-of-type(an+b) pseudo-class notation represents an element that has an+b-1 siblings with the same expanded element name before it in the document tree, for any zero or positive integer value of n, and has a parent element. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values.

img:nth-of-type(2n+1) { float: right; }
  img:nth-of-type(2n) { float: left; }

:nth-last-of-type() pseudo-class

The :nth-last-of-type(an+b) pseudo-class notation represents an element that has an+b-1 siblings with the same expanded element name after it in the document tree, for any zero or positive integer value of n, and has a parent element. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values.

body > h2:nth-of-type(n+2):nth-last-of-type(n+2)

body > h2:not(:first-of-type):not(:last-of-type)

:first-child pseudo-class

Same as :nth-child(1). The :first-child pseudo-class represents an element that is the first child of some other element.

div > p:first-child

<p> The last P before the note.</p>
  <div class="note">
     <p> The first P inside the note.</p>
  </div>

<p> The last P before the note.</p>
  <div class="note">
     <h2> Note </h2>
     <p> The first P inside the note.</p>
  </div>

* > a:first-child /* a is first child of any element */
  a:first-child /* Same (assuming a is not the root element) */

:last-child pseudo-class

Same as :nth-last-child(1). The :last-child pseudo-class represents an element that is the last child of some other element.

ol > li:last-child

:first-of-type pseudo-class

Same as :nth-of-type(1). The :first-of-type pseudo-class represents an element that is the first sibling of its type in the list of children of its parent element.

dl dt:first-of-type

<dl>
   <dt>gigogne</dt>
   <dd>
    <dl>
     <dt>fusée</dt>
     <dd>multistage rocket</dd>
     <dt>table</dt>
     <dd>nest of tables</dd>
    </dl>
   </dd>
  </dl>

:last-of-type pseudo-class

Same as :nth-last-of-type(1). The :last-of-type pseudo-class represents an element that is the last sibling of its type in the list of children of its parent element.

tr > td:last-of-type

:only-child pseudo-class

Represents an element that has a parent element and whose parent element has no other element children. Same as :first-child:last-child or :nth-child(1):nth-last-child(1), but with a lower specificity.

:only-of-type pseudo-class

Represents an element that has a parent element and whose parent element has no other element children with the same expanded element name. Same as :first-of-type:last-of-type or :nth-of-type(1):nth-last-of-type(1), but with a lower specificity.

:empty pseudo-class

The :empty pseudo-class represents an element that has no children at all. In terms of the document tree, only element nodes and content nodes (such as DOM [[DOM-LEVEL-3-CORE]] text nodes, CDATA nodes, and entity references) whose data has a non-zero length must be considered as affecting emptiness; comments, processing instructions, and other nodes must not affect whether an element is considered empty or not.

<p></p>

<foo>bar</foo>

<foo><bar>bla</bar></foo>

<foo>this is not <bar>:empty</bar></foo>

Combinators

Descendant combinator

At times, authors may want selectors to describe an element that is the descendant of another element in the document tree (e.g., "an EM element that is contained within an H1 element"). Descendant combinators express such a relationship. A descendant combinator is whitespace that separates two compound selectors. A selector of the form "A B" represents an element B that is an arbitrary descendant of some ancestor element A.

h1 em

<h1>This <span class="myclass">headline
  is <em>very</em> important</span></h1>

div * p

div p *[href]

Child combinators

A child combinator describes a childhood relationship between two elements. A child combinator is made of the "greater-than sign" (U+003E, >) character and separates two compound selectors.

body > p

div ol>li p

For information on selecting the first child of an element, please see the section on the :first-child pseudo-class above.

Sibling combinators

There are two different sibling combinators: the adjacent sibling combinator and the general sibling combinator. In both cases, non-element nodes (e.g. text between elements) are ignored when considering adjacency of elements.

Adjacent sibling combinator

The adjacent sibling combinator is made of the "plus sign" (U+002B, +) character that separates two compound selectors. The elements represented by the two compound selectors share the same parent in the document tree and the element represented by the first compound selector immediately precedes the element represented by the second one.

math + p

h1.opener + h2

General sibling combinator

The general sibling combinator is made of the "tilde" (U+007E, ~) character that separates two compound selectors. The elements represented by the two compound selectors share the same parent in the document tree and the element represented by the first compound selector precedes (not necessarily immediately) the element represented by the second one.

h1 ~ pre

<h1>Definition of the function a</h1>
  <p>Function a(x) has to be applied to all figures in the table.</p>
  <pre>function a(x) = 12x/13.5</pre>

Calculating a selector's specificity

A selector's specificity is calculated as follows:

• count the number of ID selectors in the selector (= a)
• count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= b)
• count the number of type selectors and pseudo-elements in the selector (= c)
• ignore the universal selector

The specificity of a negation or matches pseudo-class is the specificity of its most specific argument. The pseudo-class itself does not count as pseudo-class.

It would probably be better to have match-sensitive specificity, if possible. See dbaron's message.

Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity.

*               /* a=0 b=0 c=0 -> specificity =   0 */
  LI              /* a=0 b=0 c=1 -> specificity =   1 */
  UL LI           /* a=0 b=0 c=2 -> specificity =   2 */
  UL OL+LI        /* a=0 b=0 c=3 -> specificity =   3 */
  H1 + *[REL=up]  /* a=0 b=1 c=1 -> specificity =  11 */
  UL OL LI.red    /* a=0 b=1 c=3 -> specificity =  13 */
  LI.red.level    /* a=0 b=2 c=1 -> specificity =  21 */
  #x34y           /* a=1 b=0 c=0 -> specificity = 100 */
  #s12:not(FOO)   /* a=1 b=0 c=1 -> specificity = 101 */
  

Note: Repeated occurrances of the same simple selector are allowed and do increase specificity.

Note: the specificity of the styles specified in an HTML style attribute is described in CSS 2.1. [[!CSS21]].

The grammar of Selectors

Grammar

The grammar below defines the syntax of Selectors. It is globally LL(1) and can be locally LL(2) (but note that most UAs should not use it directly, since it doesn't express the parsing conventions). The format of the productions is optimized for human consumption and some shorthand notations beyond Yacc (see [[!YACC]]) are used:

• *: 0 or more
• +: 1 or more
• ?: 0 or 1
• |: separates alternatives
• [ ]: grouping

The productions are:

selectors_group
    : selector [ COMMA S* selector ]*
    ;

  selector
    : compound_selector [ combinator simple_selector_sequence ]*
    ;

  combinator
    /* combinators can be surrounded by whitespace */
    : PLUS S* | GREATER S* | TILDE S* | S+
    ;

  simple_selector_sequence
    : [ type_selector | universal ]
      [ HASH | class | attrib | pseudo | negation ]*
    | [ HASH | class | attrib | pseudo | negation ]+
    ;

  type_selector
    : [ namespace_prefix ]? element_name
    ;

  namespace_prefix
    : [ IDENT | '*' ]? '|'
    ;

  element_name
    : IDENT
    ;

  universal
    : [ namespace_prefix ]? '*'
    ;

  class
    : '.' IDENT
    ;

  attrib
    : '[' S* [ namespace_prefix ]? IDENT S*
          [ [ PREFIXMATCH |
              SUFFIXMATCH |
              SUBSTRINGMATCH |
              '=' |
              INCLUDES |
              DASHMATCH ] S* [ IDENT | STRING ] S*
          ]? ']'
    ;

  pseudo
    /* '::' starts a pseudo-element, ':' a pseudo-class */
    /* Exceptions: :first-line, :first-letter, :before and :after. */
    /* Note that pseudo-elements are restricted to one per selector and */
    /* occur only in the last compound_selector. */
    : ':' ':'? [ IDENT | functional_pseudo ]
    ;

  functional_pseudo
    : FUNCTION S* expression ')'
    ;

  expression
    /* In CSS3, the expressions are identifiers, strings, */
    /* or of the form "an+b" */
    : [ [ PLUS | '-' | DIMENSION | NUMBER | STRING | IDENT ] S* ]+
    ;

  negation
    : NOT S* negation_arg S* ')'
    ;

  negation_arg
    : type_selector | universal | HASH | class | attrib | pseudo
    ;

Lexical scanner

The following is the tokenizer, written in Flex (see [[!FLEX]]) notation. The tokenizer is case-insensitive.

The two occurrences of "\377" represent the highest character number that current versions of Flex can deal with (decimal 255). They should be read as "\4177777" (decimal 1114111), which is the highest possible code point in Unicode/ISO-10646. [[!UNICODE]]

%option case-insensitive

  ident     [-]?{nmstart}{nmchar}*
  name      {nmchar}+
  nmstart   [_a-z]|{nonascii}|{escape}
  nonascii  [^\0-\177]
  unicode   \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
  escape    {unicode}|\\[^\n\r\f0-9a-f]
  nmchar    [_a-z0-9-]|{nonascii}|{escape}
  num       [0-9]+|[0-9]*\.[0-9]+
  string    {string1}|{string2}
  string1   \"([^\n\r\f\\"]|\\{nl}|{nonascii}|{escape})*\"
  string2   \'([^\n\r\f\\']|\\{nl}|{nonascii}|{escape})*\'
  invalid   {invalid1}|{invalid2}
  invalid1  \"([^\n\r\f\\"]|\\{nl}|{nonascii}|{escape})*
  invalid2  \'([^\n\r\f\\']|\\{nl}|{nonascii}|{escape})*
  nl        \n|\r\n|\r|\f
  w         [ \t\r\n\f]*

  D         d|\\0{0,4}(44|64)(\r\n|[ \t\r\n\f])?
  E         e|\\0{0,4}(45|65)(\r\n|[ \t\r\n\f])?
  N         n|\\0{0,4}(4e|6e)(\r\n|[ \t\r\n\f])?|\\n
  O         o|\\0{0,4}(4f|6f)(\r\n|[ \t\r\n\f])?|\\o
  T         t|\\0{0,4}(54|74)(\r\n|[ \t\r\n\f])?|\\t
  V         v|\\0{0,4}(58|78)(\r\n|[ \t\r\n\f])?|\\v

  %%

  [ \t\r\n\f]+     return S;

  "~="             return INCLUDES;
  "|="             return DASHMATCH;
  "^="             return PREFIXMATCH;
  "$="             return SUFFIXMATCH;
  "*="             return SUBSTRINGMATCH;
  {ident}          return IDENT;
  {string}         return STRING;
  {ident}"("       return FUNCTION;
  {num}            return NUMBER;
  "#"{name}        return HASH;
  {w}"+"           return PLUS;
  {w}">"           return GREATER;
  {w}","           return COMMA;
  {w}"~"           return TILDE;
  ":"{N}{O}{T}"("  return NOT;
  @{ident}         return ATKEYWORD;
  {invalid}        return INVALID;
  {num}%           return PERCENTAGE;
  {num}{ident}     return DIMENSION;
  "<!--"           return CDO;
  "-->"            return CDC;

  \/\*[^*]*\*+([^/*][^*]*\*+)*\/                    /* ignore comments */

  .                return *yytext;

Profiles

Each specification using Selectors must define the subset of Selectors it allows and excludes, and describe the local meaning of all the components of that subset.

Non normative examples:

h1 a[name]

Conformance

Conformance Classes

Conformance to Selectors Level 4 is defined for three conformance classes:

selector instance

A written selector.

interpreter

A UA that interprets the semantics of a selector.

authoring tool

A UA that writes a style sheet.

A selector instance is conformant to Selectors Level 4 if it is valid according to the selector syntax rules defined in this specification.

An interpreter is conformant to Selectors Level 4 if it parses interprets selectors according to the semantics defined in Selectors Level 4 (including following the error-handling rules). However, the inability of a user agent to implement part of this specification due to the limitations of a particular device (e.g., non interactive user agents will probably not implement dynamic pseudo-classes because they make no sense without interactivity) does not imply non-conformance.

An authoring tool is conformant to Selectors Level 4 if it writes syntactically correct selectors.

Any specification reusing Selectors must contain a Profile listing the subset of Selectors it accepts or excludes, and describing any constraints it adds to the current specification.

Specifications reusing Selectors must define how to handle parsing errors. (In the case of CSS, the entire rule in which the selector is used is dropped.)

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support.

Experimental Implementations

To avoid clashes with future Selectors features, the Selectors specification reserves a prefixed syntax for proprietary extensions to Selectors. The CSS Working Group recommends that experimental implementations of features in Selectors Working Drafts also use vendor-prefixed property or value names. This avoids any incompatibilities with future changes in the draft. Once a specification reaches the Candidate Recommendation stage, implementors should implement the non-prefixed syntax for any feature they consider to be correctly implemented according to spec.

Acknowledgements

The CSS working group would like to thank everyone who contributed to the previous Selectors specifications over the years..

References

Normative References

Informative References