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

This module is an early-stage Working Draft. If you are looking for a stable Selectors specification, use Selectors 3. Read the CSS Snapshot for an overview of the CSS development process.

Table of Contents

Introduction

This section is not normative.

Selectors define the following function for elements in a tree structure:

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. This module defines Selectors Level 4.

Module Interactions

This module replaces the definitions for, and extends the set of 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, due to their close integration with the rendering model and irrelevance to other uses such as DOM queries, they will be defined in other modules.

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 and Structure

Structure and Terminology

The term selector can refer to a simple selector, compound selector, complex selector, or selector list.

A complex 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.

Determining the 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 a dollar sign ($) to one of the compound selectors in a selector. Although the element structure that the selector represents is the same with or without the dollar sign, indicating the subject in this way can change which compound selector represents the subject in that structure.

OL > LI:only-child

$OL > LI:only-child

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, for functional pseudo-classes, by a value between parentheses. White space is optionally allowed between the parentheses and the argument, but not between the pseudo-class name and the parentheses.

Pseudo-classes are allowed in all compound selectors contained in a selector. Pseudo-classes are allowed anywhere in a compound selector after the leading type selector or (possibly omitted) universal selector. Pseudo-class names are case-insensitive. Some pseudo-classes are mutually exclusive (such that a compound selector containing them, while valid, will never match anything), while others can apply simultaneously to the same element. Pseudo-classes may be dynamic, in the sense that an element can acquire or lose a pseudo-class while a user interacts with the document.

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.

Pseudo-elements

Pseudo-elements create abstractions about the document tree beyond those specified by the document language. For instance, document languages do not offer mechanisms to access the first letter or first line of an element's content. Pseudo-elements allow authors to refer to this otherwise inaccessible information. Pseudo-elements may also provide authors a way to refer to content that does not exist in the source document (e.g., the ::before and ::after pseudo-elements give access to generated content in CSS [[CSS21]]).

A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.

This :: notation was chosen in order to establish a discrimination between pseudo-classes (which subclass existing elements) and pseudo-elements (which are elements not represented in the document tree). However, for compatibility with existing style sheets, user agents must also accept the previous one-colon notation for pseudo-elements introduced in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and :after). This compatibility notation is not allowed any other pseudo-elements.

Only one pseudo-element may appear per selector, and only if the subject of the selector is the last compound selector in the selector. If present the pseudo-element must appear after the compound selector that represents the subjects of the selector.

A future version of this specification may allow multiple pseudo-elements per selector.

Syntactically, a pseudo-element may be followed by any combination of the user action pseudo-classes. Whether these pseudo-classes can match on the pseudo-element depends on the pseudo-class and pseudo-element's definition: unless otherwise-specified, none of these pseudo-classes will match on the pseudo-element.

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 selector list containing an invalid selector is invalid.

An invalid selector represents nothing.

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

Logical Combinations

Selector Lists

A comma-separated list of selectors represents the union of all elements selected by each of the individual selectors in the selector 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-any pseudo-class, :matches(X), is a functional notation taking a selector list as its 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-any 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 list 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]]

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]]

object[type^="image/"]

a[href$=".html"]

p[title*="hello"]

Case-sensitivity

By default case-sensitivity of attribute names and values in selectors depends on the document language. To match attribute values case-insensitively regardless of document language rules, the attribute selector may include the identifier i immediately inside the closing bracket (]). When this flag is present, UAs must match the attribute's value case-insensitively within the ASCII range.

[frame=hsides i] { border-style: solid none; }
    

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"). 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

The class selector is given as a full stop (. U+002E) immediately followed by an identifier. It represents an element belonging to the class identified by the identifier, as defined by the document language. For example, in [[HTML5]], [[SVG11]], and [[MATHML]] membership in a class is given by the class attribute: in these languages it is equivalent to the ~= notation applied to the local class attribute (i.e. [class~=identifier]), except that it has a higher specificity.

*.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.

Location Pseudo-classes

The hyperlink pseudo-class: '':any-link''

The :any-link pseudo-class represents an element that acts as the source anchor of a hyperlink.

Any better name suggestions for this pseudo?

The link history 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.

.footnote: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 local link pseudo-class '':local-link''

The :local-link pseudo-class allows authors to style links based on the users current location within a site and to differentiate site-internal versus site-external links. The :local-link pseudo-class represents an element that is the source anchor of a hyperlink whose target's absolute URI matches the element's own document URI. The fragment identifier of the document URI is stripped before matching against the link's URI; otherwise all portions of the URI are considered.

nav :local-link { text-decoration: none; }

The pseudo-class can also accept a non-negative integer as its sole argument, which, if the document's URI is a URL, indicates the number of path levels to match: an argument of zero represents a link element whose target is in the same domain as the document's URI, ''1'' represents a link element whose target has the same domain and first path segment, ''2'' represents a link element whose target has the same domain, first, and second path segments, etc. Path segments are portions of the URL's path that are separated by forward slashes (/). If a segment is missing from the document's URL, a pseudo-class requiring that segment to match does not match anything. Similarly if the document's URI is not a URL, the pseudo-class does not match anything. The scheme, username, password, port, query string, and fragment portions of the URL are not considered when matching against :local-link(n).

Is there such a thing as IRL? Because we do want this to work for internationalized URLs, just not URNs.

:not(:local-link(0)) { text-decoration-style: dashed; }

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 contextual reference element pseudo-class '':scope''

The :scope pseudo-class represents any element that is in the contextual reference element set. If no contextual reference element set is given, :scope is equivalent to :root.

The contextual reference element set is a (potentially empty) explicitly-specified set of elements, such as that specified by the querySelector() call in [[SELECT-API2]]. Specifications intending for this pseudo-class to match specific elements rather than the document's root element must define a contextual reference element set.

User Action Pseudo-classes

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.

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

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

a:focus
  a:focus:hover

The pointer hover pseudo-class '':hover''

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 parent of an element that is :hover is also in that state.

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

The :hover pseudo-class can apply to any pseudo-element.

The activation pseudo-class '':active''

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.

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

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

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

The input focus pseudo-class '':focus''

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 acquire :focus.

Time-linear Presentation Pseudo-classes

These pseudo-classes classify elements with respect to the currently-displayed or active position in a time-dimensional canvas, such as during speech rendering of an HTML document.

The current-element pseudo-class '':current''

The :current pseudo-class represents the innermost element, or ancestor of an element, that is currently being displayed.

Its alternate form :current(), like :matches(), takes a list of compound selectors as its argument: it represents the :current element that matches the argument or, if that does not match, the innermost ancestor of the :current element that does. (If neither the :current element nor its ancestors match the argument, then the selector does not represent anything.)

:current(p, li, dt, dd) {
  background: yellow;
}

The past-element pseudo-class '':past''

The :past pseudo-class represents any element that is defined to occur entirely prior to a :current element. If a time-based order of elements is not defined by the document language, then this represents any element that is a (possibly indirect) previous sibling of a :current element.

The future-element pseudo-class '':future''

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

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

:not(:checked)

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; }

: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 */

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

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) */

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

  :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)
  

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

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 */

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

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

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

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) */

ol > li:last-child

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>

tr > td:last-of-type

<p></p>

<foo>bar</foo>

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

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

:column(col.selected) { background: yellow; }

<table>
  <col span="2">
  <col class="selected">
  <tr><td>A <td>B <td>C
  <tr><td span="2">D <td>E
  <tr><td>F <td span="2">G
</table>

h1 em

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

div * p

div p *[href]

body > p

div ol>li p

math + p

h1.opener + h2

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>

label:matches(:hover, :focus) /for/ input,       /* association by "for" attribute */
label:matches(:hover, :focus):not([for]) input { /* association by containment */
  box-shadow: yellow 0 0 10px; }

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
    ;

%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;

h1 a[name]