Hoja de referencia de selectores CSS

CSS selectors pick the elements a rule applies to. They are also what document.querySelectorAll(), Playwright, Puppeteer and most scrapers use, so the same syntax works far beyond stylesheets.

/* every <a> inside a .card that opens in a new tab */
.card a[target='_blank'] {
  color: rebeccapurple;
}

🎯 Basic Selectors

Selector	Example	Selects
`*`	`*`	Every element
`element`	`p`	Every `<p>` element
`.class`	`.intro`	Every element with `class="intro"`
`#id`	`#firstname`	The element with `id="firstname"`
`.class1.class2`	`.name1.name2`	Elements carrying both classes
`element.class`	`p.intro`	Every `<p>` with `class="intro"`
`a, b`	`div, p`	Every `<div>` and every `<p>` (a list)

🔗 Combinators

Selector	Example	Selects
`a b`	`div p`	Every `<p>` anywhere inside a `<div>` (descendant)
`a > b`	`div > p`	Every `<p>` whose direct parent is a `<div>` (child)
`a + b`	`div + p`	The `<p>` immediately after a `<div>` (adjacent sibling)
`a ~ b`	`p ~ ul`	Every `<ul>` that follows a `<p>` under the same parent (general sibling)

🏷 Attribute Selectors

Selector	Example	Selects
`[attr]`	`[target]`	Elements that have the attribute at all
`[attr="value"]`	`[target="_blank"]`	Exact value
`[attr~="value"]`	`[title~="flower"]`	Value is one of a space-separated list
`[attr\|="value"]`	`[lang\|="en"]`	Value is `en` or starts with `en-`
`[attr^="value"]`	`a[href^="https"]`	Value starts with
`[attr$="value"]`	`a[href$=".pdf"]`	Value ends with
`[attr*="value"]`	`a[href*="docs"]`	Value contains
`[attr="value" i]`	`[type="TEXT" i]`	Case-insensitive match (add `s` instead to force case-sensitivity)

🌳 Structural Pseudo-Classes

Selector	Example	Selects
`:root`	`:root`	The document root — where custom properties usually live
`:first-child`	`p:first-child`	A `<p>` that is the first child of its parent
`:last-child`	`p:last-child`	A `<p>` that is the last child of its parent
`:only-child`	`p:only-child`	A `<p>` that is the only child
`:nth-child(n)`	`li:nth-child(2)`	The second child — also `odd`, `even`, `3n+1`
`:nth-last-child(n)`	`li:nth-last-child(2)`	Counting from the end
`:first-of-type`	`p:first-of-type`	The first `<p>` among its siblings
`:last-of-type`	`p:last-of-type`	The last `<p>` among its siblings
`:only-of-type`	`p:only-of-type`	The only `<p>` among its siblings
`:nth-of-type(n)`	`p:nth-of-type(2)`	The second `<p>` among its siblings
`:nth-last-of-type(n)`	`p:nth-last-of-type(2)`	Same, counting from the end
`:empty`	`p:empty`	No children at all, text included

🖱 State & Interaction

Selector	Example	Selects
`:link`	`a:link`	Unvisited links
`:visited`	`a:visited`	Visited links
`:hover`	`a:hover`	The element under the pointer
`:active`	`a:active`	The element being clicked
`:focus`	`input:focus`	The focused element
`:focus-visible`	`button:focus-visible`	Focused and the browser thinks a ring belongs there (keyboard)
`:focus-within`	`form:focus-within`	An element containing the focused element
`:target`	`#news:target`	The element the URL fragment points at
`:fullscreen`	`:fullscreen`	The element displayed full-screen

📝 Form Pseudo-Classes

Selector	Example	Selects
`:checked`	`input:checked`	Checked checkboxes, radios and selected options
`:default`	`input:default`	The default control in a form
`:disabled`	`input:disabled`	Disabled controls
`:enabled`	`input:enabled`	Controls that are not disabled
`:required`	`input:required`	Controls carrying `required`
`:optional`	`input:optional`	Controls without `required`
`:valid`	`input:valid`	Controls whose value passes validation
`:invalid`	`input:invalid`	Controls whose value fails validation
`:user-valid`	`input:user-valid`	Valid, but only after the user has interacted
`:user-invalid`	`input:user-invalid`	Invalid, but only after the user has interacted
`:in-range`	`input:in-range`	Value inside `min`/`max`
`:out-of-range`	`input:out-of-range`	Value outside `min`/`max`
`:indeterminate`	`input:indeterminate`	Neither checked nor unchecked
`:read-only`	`input:read-only`	Not editable by the user
`:read-write`	`input:read-write`	Editable by the user
`:placeholder-shown`	`input:placeholder-shown`	The placeholder is currently visible

🧩 Functional Pseudo-Classes

Selector	Example	Selects
`:is()`	`:is(h1, h2, h3) > code`	Any of the listed selectors — takes the highest specificity of the list
`:where()`	`:where(h1, h2) a`	Same matching as `:is()`, but contributes zero specificity
`:not()`	`li:not(.done, .hidden)`	Elements that match none of the listed selectors
`:has()`	`article:has(> img)`	A parent that contains a match — the “parent selector”
`:lang()`	`p:lang(it)`	Elements in a given language
`:nth-child(An+B of S)`	`li:nth-child(2n of .visible)`	Counts only the siblings matching `S`

✨ Pseudo-Elements

Pseudo-elements use :: (double colon) and target a part of an element rather than the element itself.

Selector	Example	Targets
`::before`	`p::before`	Generated content in front of the content
`::after`	`p::after`	Generated content after the content
`::first-letter`	`p::first-letter`	The first letter
`::first-line`	`p::first-line`	The first rendered line
`::marker`	`li::marker`	The bullet or number of a list item
`::selection`	`::selection`	The part the user has selected
`::placeholder`	`input::placeholder`	The placeholder text
`::backdrop`	`dialog::backdrop`	The layer behind a modal or full-screen element
`::file-selector-button`	`input::file-selector-button`	The button of a file input

ℹ️ ::before and ::after need a content property — even content: "" — before they render.

📊 Specificity

Specificity is compared as three numbers, ID – class – type, highest wins; a later rule only wins against an equal score.

Selector kind	Weight	Example
Inline `style` attribute	overrides everything below	`style="…"`
ID	`1-0-0`	`#header`
Class, attribute, pseudo-class	`0-1-0`	`.card`, `[href]`, `:hover`
Element, pseudo-element	`0-0-1`	`p`, `::before`
`*`, combinators, `:where()`	`0-0-0`	`*`, `>`, `:where(.a)`

#nav .item a      /* 1-1-1 */
.menu .item a     /* 0-2-1 — loses to the line above */
:where(#nav) a    /* 0-0-1 — :where() zeroes out what is inside it */

💡 Reach for :where() in library or reset styles so consumers can override them without an !important arms race.