Selectores CSS

Los selectores definen sobre qué elementos se aplicará un conjunto de reglas.

Selectores Básicos

Selectores de Tipo

Este selector básico selecciona todos los elementos que coinciden con el nombre dado.
Sintáxis: eltname
Ejemplo: input aplicará a cualquier elemento <input>.

Selectores de Clase

Este selector básico aplica a aquellos elementos basados en el valor de su atributo class.
Sintáxis: .classname
Ejemplo: .index will match any element that has the class index (likely defined by a class="index" attribute or similar).

ID selectors

This basic selector chooses nodes based on the value of its id attribute. There should be only one element with a given ID in a document.
Syntax: #idname
Example: #toc will match the element that has the id toc (likely defined by a id="toc" attribute or similar).

Universal selectors

This basic selector chooses all nodes. It also exists in a one-namespace only and in an all-namespace variant too.
Syntax: * ns|* *|*
Example: * will match all the elements of the document.

Attribute selectors

This basic selector chooses nodes based on the value of one of its attributes.
Syntax: [attr] [attr=value] [attr~=value] [attr|=value] [attr^=value] [attr$=value] [attr*=value]
Example: [autoplay] will match all the elements that have the autoplay attribute set (to any value).

Combinators

Adjacent sibling selectors

The '+' combinator selects nodes that immediately follow the former specified element.
Syntax: A + B
Example: ul + li will match any <li> that immediately follows a <ul>.

General sibling selectors

The '~' combinator selects nodes that follow (not necessarily immediately) the former specified element, if both elements shared the same parent.
Syntax: A ~ B
Example: p ~ span will match all <span> elements that follow a <p> element inside the same element.

Child selectors

The '>' combinator selects nodes that are direct children of the former specified element.
Syntax: A > B
Example: ul > li will match all <li> elements that are inside a <ul> element.

Descendant selectors

The ' ' combinator selects nodes that are children (not necessary direct children) of the former specified element.
Syntax: A B
Example: div span will match any <span> element that is inside a <div> element.

Pseudo-elements

Pseudo-elements are abstractions of the tree representing entities beyond what HTML does. For example, HTML doesn't have an element describing the first letter or line of a paragraph, or the marker of a list. Pseudo-elements represent these entities and allow CSS rules to be associated with them. that way, these entitities can be styled independently.

Pseudo-classes

Pseudo-classes allow to select elements based on information that is not contained in the document tree like a state or that is particularly complex to extract. E.g. they match if a link has been previously visited or not.

Specifications

Browser compatibility