Selectors Level 4

1. Introduction

This section is not normative.

A selector is a boolean predicate that takes an element in a tree structure and tests whether the element matches the selector or not.

These expressions may be used for many things:

• directly on an element to test whether it matches some criteria, such as in the element.matches() function defined in [DOM]
• applied to an entire tree of elements to filter it into a set of elements that match the criteria, such as in the document.querySelectorAll() function defined in [DOM] or the selector of a CSS style rule.
• used "in reverse" to generate markup that would match a given selector, such as in HAML or Emmet.

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.

1.1. Module Interactions

This module replaces the definitions of and extends the set of selectors defined for CSS in [SELECT] 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.

2. 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:

Note: Some Level 4 selectors (noted above as "3-UI") were introduced in [CSS3UI].

3. Selector Syntax and Structure

3.1. Structure and Terminology

A selector represents a particular pattern of element(s) in a tree structure. The term selector can refer to a simple selector, compound selector, complex selector, or selector list. The subject of a selector is any element that selector is defined to be about; that is, any element matching that selector.

A simple selector is a single condition on an element. A type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class is a simple selector. (It is represented by <simple-selector> in the selectors grammar.) A given element is said to match a simple selector when that simple selector, as defined in this specification and in accordance with the document language, accurately describes the element.

A compound selector is a sequence of simple selectors that are not separated by a combinator, and represents a set of simultaneous conditions on a single element. If it contains a type selector or universal selector, that selector must come first in the sequence. Only one type selector or universal selector is allowed in the sequence. (A compound selector is represented by <compound-selector> in the selectors grammar.) A given element is said to match a compound selector when it matches all simple selectors in the compound selector.

Note: As whitespace represents the descendant combinator, no whitespace is allowed between the simple selectors in a compound selector.

A pseudo-compound selector is a pseudo-element selector, optionally followed by additional pseudo-class selectors, and optionally preceded by a compound selector or another pseudo-compound selector, without any combinators. (A pseudo-compound selector is represented by <pseudo-compound-selector> in the selectors grammar.) A pseudo-element matches a pseudo-compound selector when it has the specified pseudo-element name, matches the additional conditions represented by any pseudo-classes, and has an originating element represented by the adjacent preceding selector. If there is no adjacent preceding selector, the universal selector is assumed. (For example, .foo ::before is equivalent to .foo *::before, and distinct from .foo::before.)

Note: A pseudo-compound selector is not a compound selector, and can’t be used in places that expect a compound selector only. Pseudo-compound selectors act as if they carry a combinator with themselves, expressing their relationship with their originating element, just as the > combinator expresses a relationship with a parent element.

A combinator is a condition of relationship between two elements represented by the compound selectors on either side. Combinators in Selectors Level 4 include: the descendant combinator (white space), the child combinator (U+003E, >), the next-sibling combinator (U+002B, +), and the subsequent-sibling combinator (U+007E, ~). Two given elements are said to match a combinator when the condition of relationship between these elements is true.

A complex selector is a sequence of one or more compound selectors and/or pseudo-compound selectors, with compound selectors separated by combinators. It represents a set of simultaneous conditions on a set of elements in the particular relationships described by its combinators. (Complex selectors are represented by <complex-selector> in the selectors grammar.) A given element or pseudo-element is said to match a complex selector when it matches the final compound/pseudo-compound selector in the sequence, and every preceding unit of the sequence also matches an element or pseudo-element, with the correct relationship between consecutive units as expressed by the combinators separating them (or, for pseudo-compound selectors, the correct originating element relationship).

A list of simple/compound/complex selectors is a comma-separated list of simple, compound, or complex selectors. This is also called just a selector list when the type is either unimportant or specified in the surrounding prose; if the type is important and unspecified, it defaults to meaning a list of complex selectors. (See § 4.1 Selector Lists for additional information on selector lists and the various <*-selector-list> productions in the grammar for their formal syntax.) A given element is said to match a selector list when it matches any (at least one) of the selectors in that selector list.

3.2. Data Model

Selectors are evaluated against an element tree such as the DOM. [DOM] Within this specification, this may be referred to as the "document tree" or "source document".

Each element may have any of the following five aspects, which can be selected against, all of which are matched as strings:

• The element’s type (also known as its tag name).
• The element’s namespace.
• An ID.
• Classes (named groups) to which it belongs.
• Attributes, which are name-value pairs.

Many of the selectors depend on the semantics of the document language (i.e. the language and semantics of the document tree) and/or the semantics of the host language (i.e. the language that is using selectors syntax). For example, the :lang() selector depends on the document language (e.g. HTML) to define how an element is associated with a language. As a slightly different example, the ::first-line pseudo-element depends on the host language (e.g. CSS) to define what a ::first-line pseudo-element represents and what it can do.

3.2.1. Featureless Elements

While individual elements may lack any of the above features, some elements are featureless. A featureless element does not match any selector at all, except:

• simple selectors it is explicitly defined to match

• compound selectors, if all contained simple selectors are allowed to match it

• complex selectors, if the compound selector targeting the subject is allowed to match it

• selector lists, if at least one selector in the list is allowed to match it

• logical combination pseudo-classes, if their argument selector is allowed to match it

• the :has() pseudo-class, if and only if the compound selector it’s part of contains at least one other simple selector that’s allowed to match it.

If a selector would otherwise match a featureless element, except for the existence of the default namespace [CSS-NAMESPACES-3] (because featureless elements do not have a namespace unless otherwise defined), the default namespace does not prevent the match.

3.3. Scoped Selectors

Some host applications may choose to scope selectors to a particular subtree or fragment of the document, The root of the scoping subtree is called the scoping root.

When a selector is scoped, it matches an element only if the element is a descendant of the scoping root. (The rest of the selector can match unrestricted; it’s only the final matched elements that must be within the scope.)

3.4. Relative Selectors

Certain contexts may accept relative selectors, which are a shorthand for selectors that represent elements relative to one or more relative selector anchor elements. Relative selectors begin with a combinator, with a selector representing the anchor element implied at the start of the selector. (If no combinator is present, the descendant combinator is implied.)

Relative selectors are represented by <relative-selector> in the selectors grammar, and lists of them by <relative-selector-list>.

3.5. Pseudo-classes

Pseudo-classes are simple selectors that permit selection based on information that lies outside of the document tree or that can be awkward or impossible to express using the other simple selectors. They can also be dynamic, in the sense that an element can acquire or lose a pseudo-class while a user interacts with the document, without the document itself changing. Pseudo-classes do not appear in or modify the document source or document tree.

The syntax of a pseudo-class consists of a ":" (U+003A COLON) followed by the name of the pseudo-class as a CSS identifier, and, in the case of a functional pseudo-class, a pair of parentheses containing its arguments.

For example, :valid is a regular pseudo-class, and :lang() is a functional pseudo-class.

Like all CSS keywords, pseudo-class names are ASCII case-insensitive. No white space is allowed between the colon and the name of the pseudo-class, nor, as usual for CSS syntax, between a functional pseudo-class’s name and its opening parenthesis (which thus form a CSS function token). Also as usual, white space is allowed around the arguments inside the parentheses of a functional pseudo-class unless otherwise specified.

Like other simple selectors, pseudo-classes are allowed in all compound selectors contained in a selector, and must follow the type selector or universal selector, if present.

Note: 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.

3.6. Pseudo-elements

Similar to how certain pseudo-classes represent additional state information not directly present in the document tree, a pseudo-element represents an element not directly present in the document tree. They are used to create abstractions about the document tree beyond those provided by the document tree. For example, pseudo-elements can be used to select portions of the document that do not correspond to a document-language element (including such ranges as don’t align to element boundaries or fit within its tree structure); that represent content not in the document tree or in an alternate projection of the document tree; or that rely on information provided by styling, layout, user interaction, and other processes that are not reflected in the document tree.

Like pseudo-classes, pseudo-elements do not appear in or modify the document source or document tree. Accordingly, they also do not affect the interpretation of structural pseudo-classes or other selectors pertaining to their originating element or its tree.

The host language defines which pseudo-elements exist, their type, and their abilities. Pseudo-elements that exist in CSS are defined in [CSS21] (Level 2), [SELECT] (Level 3), and [CSS-PSEUDO-4] (Level 4).

3.6.1. Syntax

The syntax of a pseudo-element is "::" (two U+003A COLON characters) followed by the name of the pseudo-element as an identifier, and, in the case of a functional pseudo-element, a pair of parentheses containing its arguments. Pseudo-element names are ASCII case-insensitive. No white space is allowed between the two colons, or between the colons and the name.

Because CSS Level 1 and CSS Level 2 conflated pseudo-elements and pseudo-classes by sharing a single-colon syntax for both, user agents must also accept the previous one-colon notation for the Level 1 & 2 pseudo-elements (::before, ::after, ::first-line, and ::first-letter). This compatibility notation is not allowed for any other pseudo-elements. However, as this syntax is deprecated, authors should use the Level 3+ double-colon syntax for these pseudo-elements.

Pseudo-elements are featureless, and so can’t be matched by any other selector.

3.6.2. Binding to the Document Tree

Pseudo-elements do not exist independently in the tree: they are always bound to another element on the page, called their originating element. Syntactically, a pseudo-element immediately follows the compound selector representing its originating element. If this compound selector is omitted, it is assumed to be the universal selector *.

When a pseudo-element is encountered in a selector, the part of the selector before the pseudo-element selects the originating element for the pseudo-element; the part of the selector after it, if any, applies to the pseudo-element itself. (See below.)

3.6.3. Pseudo-classing Pseudo-elements

Certain pseudo-elements may be immediately followed by any combination of certain pseudo-classes, in which case the pseudo-element is represented only when it is in the corresponding state. This specification allows any pseudo-element to be followed by any combination of the logical combination pseudo-classes and the user action pseudo-classes. Other specifications may allow additional pseudo-classes to be attached to particular pseudo-elements. Combinations that are not explicitly allowed are invalid selectors.

Note: The logical combination pseudo-classes pass any restrictions on validity of selectors at their position to their arguments.

3.6.4. Sub-pseudo-elements

Some pseudo-elements are able to be the originating element of other pseudo-elements, which are defined as the sub-pseudo-elements of this originating pseudo-element. For example, when ::before is given a list-item display type, it becomes the originating pseudo-element of its ::before::marker sub-pseudo-element.

Where disambiguation is needed, the term ultimate originating element refers to the real (non-pseudo) element from which a pseudo-element originates.

Unless the corresponding sub-pseudo-element is explicitly defined to exist in another specification, pseudo-element selectors are not valid when compounded to another pseudo-element selector. So, for example, ::before::before is an invalid selector, but ::before::marker is valid (in implementations that support the ::before::marker sub-pseudo-element).

3.6.5. Internal Structure

Some pseudo-elements are defined to have internal structure. These pseudo-elements may be followed by child/descendant combinators to express those relationships. Selectors containing combinators after the pseudo-element are otherwise invalid.

Note: A future specification may expand the capabilities of existing pseudo-elements, so some of these currently-invalid selectors (e.g. ::first-line :any-link) may become valid in the future.

The children of such pseudo-elements can simultaneously be children of other elements, too. However, at least in CSS, their rendering must be defined so as to maintain the tree-ness of the box tree.

3.7. Characters and case sensitivity

All Selectors syntax is ASCII case-insensitive (i.e. [a-z] and [A-Z] are equivalent), except for the parts that are not under the control of Selectors: specifically, the case-sensitivity of document language element names, attribute names, and attribute values depends on the document language.

Case sensitivity of namespace prefixes is defined in [CSS3NAMESPACE]. Case sensitivity of language ranges is defined in the :lang() section.

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

Code points in Selectors can be escaped with a backslash according to the same escaping rules as CSS. [CSS21] Note that escaping a code point “cancels out” any special meaning it may have in Selectors. For example, the selector #foo>a contains a combinator, but #foo\>a instead selects an element with the id foo>a.

3.8. Declaring Namespace Prefixes

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]

3.9. Invalid Selectors and Error Handling

User agents must observe the rules for handling invalid selectors:

• a parsing error in a selector, e.g. an unrecognized token or a token which is not allowed at the current parsing point (see overall § 16 Grammar and per-selector syntax definitions), causes that selector to be invalid.
• 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 empty selector, i.e. one that contains no compound selector, is invalid.

Note: Consistent with CSS’s forwards-compatible parsing principle, UAs must treat as invalid any pseudo-classes, pseudo-elements, combinators, or other syntactic constructs for which they have no usable level of support. See Partial implementations.

An invalid selector represents, and therefore matches, nothing.

3.10. Legacy Aliases

Some selectors have a legacy selector alias. This is a name which, at parse time, is converted to the standard name (and thus does not appear anywhere in any object model representing the selector).

4. Logical Combinations

Selector logic can be manipulated by compounding (logical AND), selector lists (logical OR), and the logical combination pseudo-classes :is(), :where(), and :not(). The logical combination pseudo-classes are allowed anywhere that any other pseudo-classes are allowed, but pass any restrictions to their arguments. (For example, if only compound selectors are allowed, then only compound selectors are valid within an :is().)

Note: Since inside :is() and :where() invalid arguments are dropped without invaliding the pseudo-class itself, selector arguments that are invalidated by contextual restrictions likewise do not invalidate the :is() pseudo-class itself.

4.1. 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 selector list 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 } 

4.2. The Matches-Any Pseudo-class: :is()

The matches-any pseudo-class, :is(), is a functional pseudo-class taking a <forgiving-selector-list> as its sole argument.

If the argument, after parsing, is an empty list, the pseudo-class is valid but matches nothing. Otherwise, the pseudo-class matches any element that matches any of the selectors in the list.

Note: The specificity of the :is() pseudo-class is replaced by the specificity of its most specific argument. Thus, a selector written with :is() does not necessarily have equivalent specificity to the equivalent selector written without :is() For example, if we have :is(ul, ol, .list) > [hidden] and ul > [hidden], ol > [hidden], .list > [hidden] a [hidden] child of an ol matches the first selector with a specificity of (0,2,0) whereas it matches the second selector with a specificity of (0,1,1). See § 15 Calculating a selector’s specificity.

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

Default namespace declarations do not affect the compound selector representing the subject of any selector within a :is() pseudo-class, unless that compound selector contains an explicit universal selector or type selector.

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

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

As previous drafts of this specification used the name :matches() for this pseudo-class, UAs may additionally implement this obsolete name as a legacy selector alias for :is() if needed for backwards-compatibility.

4.3. The Negation (Matches-None) Pseudo-class: :not()

The negation pseudo-class, :not(), is a functional pseudo-class taking a <complex-real-selector-list> as an argument. It represents an element that is not represented by its argument.

Note: In Selectors Level 3, only a single simple selector was allowed as the argument to :not().

Note: The specificity of the :not() pseudo-class is replaced by the specificity of the most specific selector in its argument; thus it has the exact behavior of :not(:is(argument)). See § 15 Calculating a selector’s specificity.

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)

As with :is(), default namespace declarations do not affect the compound selector representing the subject of any selector within a :not() pseudo-class, unless that compound selector contains an explicit universal selector or type selector. (See :is() for examples.)

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

4.4. The Specificity-adjustment Pseudo-class: :where()

The Specificity-adjustment pseudo-class, :where(), is a functional pseudo-class with the same syntax and functionality as :is(). Unlike :is(), neither the :where() pseudo-class, nor any of its arguments, contribute to the specificity of the selector—​its specificity is always zero.

This is useful for introducing filters in a selector while keeping the associated style declarations easy to override.

a:not(:hover) {
  text-decoration: none;
}

nav a {
  /* Has no effect */
  text-decoration: underline;
}

a:where(:not(:hover)) {
  text-decoration: none;
}

nav a {
  /* Works now! */
  text-decoration: underline;
}

Note: Future levels of Selectors may introduce an additional argument to explicitly set the specificity of that instance of the pseudo-class.

4.5. The Relational Pseudo-class: :has()