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()

The relational pseudo-class, :has(), is a functional pseudo-class taking a <relative-selector-list> as an argument. It represents an element if any of the relative selectors would match at least one element when anchored against this element.

The :has() pseudo-class cannot be nested; :has() is not valid within :has(). Also, unless explicitly defined as a :has-allowed pseudo-element, pseudo-elements are not valid selectors within :has(). (This specification does not define any :has-allowed pseudo-elements, but other specifications may do so.)

Note: Pseudo-elements are generally excluded from :has() because many of them exist conditionally, based on the styling of their ancestors, so allowing these to be queried by :has() would introduce cycles.

Note: Since :has() takes a <relative-selector-list>, its arguments are inherently complex selectors (because they start, perhaps implicitly, with a combinator). This means :has() cannot be used in contexts that don’t allow complex selectors; its arguments will be guaranteed to be invalid.

a:has(> img)

dt:has(+ dt)

section:not(:has(h1, h2, h3, h4, h5, h6))

section:has(:not(h1, h2, h3, h4, h5, h6))

5. Elemental selectors

5.1. Type (tag name) selector

A type selector is the name of a document language element type, and represents an instance of that element type in the document tree.

A type selector is written as a CSS qualified name: an identifier with an optional namespace prefix. [CSS3NAMESPACE] (See § 5.3 Namespaces in Elemental Selectors.)

5.2. Universal selector

The universal selector is a special type selector, that represents an element of any element type.

It is written as a CSS qualified name with an asterisk (* U+002A) as the local name. Like a type selector, the universal selector can be qualified by a namespace, restricting it to only elements belonging to that namespace, and is affected by a default namespace as defined in § 5.3 Namespaces in Elemental Selectors.

Unless an element is featureless, the presence of a universal selector has no effect on whether the element matches the selector. (Featureless elements do not match any selector, including the universal selector.)

The universal selector follows the same syntax rules as other type selectors: only one can appear per compound selector, and it must be the first simple selector in the compound selector.

Note: In some cases, adding a universal selector can make a selector easier to read, even though it has no effect on the matching behavior. For example, div :first-child and div:first-child are somewhat difficult to tell apart at a quick glance, but writing the former as div *:first-child makes the difference obvious.

5.3. Namespaces in Elemental Selectors

Type selectors and universal 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.) It has the following meaning in each form:

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 }

If a default namespace is declared, compound selectors without type selectors in them still only match elements in that default namespace.

@namespace url("http://example.com/foo");

.special { ... }

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

5.4. The Defined Pseudo-class: :defined

In some host languages, elements can have a distinction between being “defined”/“constructed” or not. The :defined pseudo-class matches elements that are fully defined, as dictated by the host language.

If the host language does not have this sort of distinction, all elements in it match :defined.

p:defined { ... }

custom-element { visibility: hidden }

custom-element:defined { visibility: visible }

6. Attribute selectors

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

Add comma-separated syntax for multiple-value matching? e.g. [rel ~= next, prev, up, first, last]

6.1. 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 <ident-token>s or <string-token>s. [CSS3SYN]

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]

6.2. 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 <ident-token>s or <string-token>s.

object[type^="image/"] 

a[href$=".html"] 

p[title*="hello"] 

6.3. Case-sensitivity

By default case-sensitivity of attribute names and values in selectors depends on the document language.

To match attribute values ASCII case-insensitively regardless of document language rules, the attribute selector may include the identifier i before the closing bracket (]). When this flag is present, UAs must match the attribute’s value ASCII case-insensitively (i.e. [a-z] and [A-Z] are considered equivalent).

Alternately, the attribute selector may include the identifier s before the closing bracket (]); in this case the UA must match the value case-sensitively, with “identical to” semantics [INFRA], regardless of document language rules.

Like the rest of Selectors syntax, the i and s identifiers themselves are ASCII case-insensitive.

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

[type="a" s] { list-style: lower-alpha; }
[type="A" s] { list-style: upper-alpha; }

Note: Some document models normalize case-insensitive attribute values at parse time such that checking if a string is case-sensitive matching is impossible. Case-sensitive matching via s flags is only possible in systems that preserve the original case.

6.4. 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 }

6.5. 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 behavior 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...*/ }

6.6. 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]).

*.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.

When matching against a document which is in quirks mode, class names must be matched ASCII case-insensitively; class selectors are otherwise case-sensitive, only matching class names they are identical to. [INFRA]

6.7. 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. Which attribute on an element is considered the “ID attribute” is defined by the document language.

An ID selector consists of a “number sign” (U+0023, #) immediately followed by the ID value, which must be a CSS identifier. An ID selector represents an element instance that has an identifier that matches the identifier in the ID selector. (It is possible in non-conforming documents for multiple elements to match a single ID selector.)

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, they 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.

When matching against a document which is in quirks mode, IDs must be matched ASCII case-insensitively; ID selectors are otherwise case-sensitive, only matching IDs they are identical to. [INFRA]

7. Linguistic Pseudo-classes

7.1. The Directionality Pseudo-class: :dir()

The :dir() pseudo-class allows the author to write selectors that represent an element based on its directionality as determined by the document language. For example, [HTML5] defines how to determine the directionality of an element, based on a combination of the dir attribute, the surrounding text, and other factors. As another example, the its:dir and dirRule element of the Internationalization Tag Set [ITS20] are able to define the directionality of an element in [XML10].

The :dir() pseudo-class does not select based on stylistic states—for example, the CSS direction property does not affect whether it matches.

The pseudo-class :dir(ltr) represents an element that has a directionality of left-to-right (ltr). The pseudo-class :dir(rtl) represents an element that has a directionality of right-to-left (rtl). The argument to :dir() must be a single identifier, otherwise the selector is invalid. White space is optionally allowed between the identifier and the parentheses. Values other than ltr and rtl are not invalid, but do not match anything. (If a future markup spec defines other directionalities, then Selectors may be extended to allow corresponding values.)

The difference between :dir(C) and [dir=C] is that [dir=C] only performs a comparison against a given attribute on the element, while the :dir(C) pseudo-class uses the UAs knowledge of the document’s semantics to perform the comparison. For example, in HTML, the directionality of an element inherits so that a child without a dir attribute will have the same directionality as its closest ancestor with a valid dir attribute. As another example, in HTML, an element that matches [dir=auto] will match either :dir(ltr) or :dir(rtl) depending on the resolved directionality of the elements as determined by its contents. [HTML5]

7.2. The Language Pseudo-class: :lang()

If the document language specifies how the (human) content language of an element is determined, it is possible to write selectors that represent an element based on its content language. The :lang() pseudo-class, which accepts a comma-separated list of one or more language ranges, represents an element whose content language is one of the languages listed in its argument. Each language range in