CSS

===Selector===
{{Redirect|CSS class|non-CSS use of element classes in HTML|class attribute (HTML)}}

In CSS, ''selectors'' declare which part of the markup a style applies to by matching tags and attributes in the markup itself.

====Selector types====
Selectors may apply to the following:
*all [[HTML element|elements]] of a specific type, e.g. the second-level headers [[HTML element#Basic text|h2]]
*elements specified by [[HTML attribute|attribute]], in particular:
**''id'': an identifier unique within the document, denoted in the selector language by a [[number sign|hash]] prefix e.g. {{Code|code=#id}}
**''class'': an identifier that can annotate multiple elements in a document, denoted by a [[period (punctuation)|dot]] prefix e.g. {{Code|code=.classname}} (the phrase "CSS class", although sometimes used, is a misnomer, as element classes—specified with the [[class attribute (HTML)|HTML class attribute]]—is a markup feature that is distinct from browsers' CSS subsystem and the related W3C/WHATWG standards work on document styles; see [[Resource Description Framework|RDF]] and [[microformat]]s for the origins of the "class" system of the Web content model)
*elements depending on how they are placed relative to others in the [[Document Object Model|document tree]].

Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens, and underscores. A class may apply to any number of instances of any element. An ID may only be applied to a single element.

====Pseudo-classes====
''Pseudo-classes'' are used in CSS selectors to permit formatting based on information that is not contained in the document tree. 

One example of a widely used pseudo-class is {{code|lang=css|:hover}}, which identifies content only when the user "points to" the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in {{code|lang=css|code=a:hover}} or {{code|lang=css|code=#elementid:hover}}. 

A pseudo-class classifies document elements, such as {{code|lang=css|code=:link}} or {{code|lang=css|code=:visited}}, whereas a ''pseudo-element'' makes a selection that may consist of partial elements, such as {{code|lang=css|code=::first-line}} or {{code|lang=css|code=::first-letter}}.<ref>{{cite web |url=https://www.w3.org/TR/CSS21/selector.html#pseudo-elements |title=W3C CSS2.1 specification for pseudo-elements and pseudo-classes |publisher=World Wide Web Consortium |date=7 June 2011 |access-date=30 April 2012 |archive-url=https://web.archive.org/web/20120430011514/https://www.w3.org/TR/CSS21/selector.html#pseudo-elements |archive-date=30 April 2012 |url-status=live }}</ref> Note the distinction between the double-colon notation used for pseudo-elements and the single-colon notation used for pseudo-classes.

====Combinators====
Multiple simple selectors may be joined using combinators to specify elements by location, element type, id, class, or any combination thereof.<ref>{{cite web |url-status=live |url=https://www.w3.org/TR/CSS21/selector.html |archive-url=https://web.archive.org/web/20060423174213/https://www.w3.org/TR/CSS21/selector.html |archive-date=2006-04-23 |title=Selectors |publisher=W3C |work=Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification}}</ref> The order of the selectors is important. For example, <syntaxhighlight inline lang="css" class=nowrap>
div .myClass {color: red;}</syntaxhighlight> applies to all elements of class myClass that are inside div elements, whereas <syntaxhighlight inline lang="css" class=nowrap>.myClass div {color: red;}</syntaxhighlight> applies to all div elements that are inside elements of class myClass. This is not to be confused with concatenated identifiers such as <syntaxhighlight inline="" lang="css" class="nowrap">
div.myClass {color: red;}</syntaxhighlight> which applies to div elements of class myClass.

====Summary of selector syntax====
The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it.<ref>{{cite web |url=https://www.w3.org/TR/selectors/ |title=Selectors Level 3 |publisher=W3C |access-date=2014-05-30 |archive-url=https://web.archive.org/web/20140603165900/https://www.w3.org/TR/selectors/ |archive-date=2014-06-03 |url-status=live }}</ref>
{| class="wikitable"
|-
! Pattern !! Matches !! First defined<br/>in CSS level
|-
| {{code|2=css|1=E}} || an element of type E || 1
|-
| {{code|2=css|1=E:link}} || an E element that is the source anchor of a hyperlink whose target is either not yet visited (:link) or already visited (:visited)  || 1
|-
| {{code|2=css|1=E:active}} || an E element during certain user actions || 1
|-
| {{code|2=css|1=E::first-line}} || the first formatted line of an E element || 1
|-
| {{code|2=css|1=E::first-letter}} || the first formatted letter of an E element || 1
|-
| {{code|2=css|1=.c}} || all elements with class="c" || 1
|-
| {{code|2=css|1=#myid}} || the element with id="myid" || 1
|-
| {{code|2=css|1=E.warning}} || an E element whose class is "warning" (the document language specifies how class is determined) || 1
|-
| {{code|2=css|1=E#myid}} || an E element with ID equal to "myid" || 1
|-
| {{code|2=css|1=.c#myid}} || the element with class="c" and ID equal to "myid" || 1
|-
| {{code|2=css|1=E F}} || an F element descendant of an E element || 1
|-
| {{code|2=css|1=*}} || any element || 2
|-
| {{code|2=css|1=E[foo]}} || an E element with a "foo" attribute || 2
|-
| {{code|2=css|1=E[foo="bar"]}} || an E element whose "foo" attribute value is exactly equal to "bar" || 2
|-
| {{code|2=css|1=E[foo~="bar"]}} || an E element whose "foo" attribute value is a list of whitespace-separated values, one of which is exactly equal to "bar" || 2
|-
| {{code|2=css|1=E<nowiki>[foo|="en"]</nowiki>}} || an E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en" || 2
|-
| {{code|2=css|1=E:first-child}} || an E element, first child of its parent || 2
|-
| {{code|2=css|1=E:lang(fr)}} || an element of type E in language "fr" (the document language specifies how language is determined) || 2
|-
| {{code|2=css|1=E::before}} || generated content before an E element's content || 2
|-
| {{code|2=css|1=E::after}} || generated content after an E element's content || 2
|-
| {{code|2=css|1=E > F}} || an F element child of an E element || 2
|-
| {{code|2=css|1=E + F}} || an F element immediately preceded by an E element || 2
|-
| {{code|2=css|1=E[foo^="bar"]}} || an E element whose "foo" attribute value begins exactly with the string "bar" || 3
|-
| {{code|2=css|1=E[foo$="bar"]}} || an E element whose "foo" attribute value ends exactly with the string "bar" || 3
|-
| {{code|2=css|1=E[foo*="bar"]}} || an E element whose "foo" attribute value contains the substring "bar" || 3
|-
| {{code|2=css|1=E:root}} || an E element, root of the document || 3
|-
| {{code|2=css|1=E:nth-child(n)}} || an E element, the n-th child of its parent || 3
|-
| {{code|2=css|1=E:nth-last-child(n)}} || an E element, the n-th child of its parent, counting from the last one || 3
|-
| {{code|2=css|1=E:nth-of-type(n)}} || an E element, the n-th sibling of its type || 3
|-
| {{code|2=css|1=E:nth-last-of-type(n)}} || an E element, the n-th sibling of its type, counting from the last one || 3
|-
| {{code|2=css|1=E:last-child}} || an E element, last child of its parent || 3
|-
| {{code|2=css|1=E:first-of-type}} || an E element, first sibling of its type || 3
|-
| {{code|2=css|1=E:last-of-type}} || an E element, last sibling of its type || 3
|-
| {{code|2=css|1=E:only-child}} || an E element, only child of its parent || 3
|-
| {{code|2=css|1=E:only-of-type}} || an E element, only sibling of its type || 3
|-
| {{code|2=css|1=E:empty}} || an E element that has no children (including text nodes) || 3
|-
| {{code|2=css|1=E:target}} || an E element being the target of the referring URI || 3
|-
| {{code|2=css|1=E:enabled}} || a user interface element E that is enabled || 3
|-
| {{code|2=css|1=E:disabled}} || a user interface element E that is disabled || 3
|-
| {{code|2=css|1=E:checked}} || a user interface element E that is checked (for instance a radio button or checkbox) || 3
|-
| {{code|2=css|1=E:not(s)}} || an E element that does not match simple selector s || 3
|-
| {{code|2=css|1=E ~ F}} || an F element preceded by an E element || 3
|}