CSS Selectors Level 3
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| CSS Selectors Level 3 | |
|---|---|
| Name | CSS Selectors Level 3 |
| Developer | World Wide Web Consortium |
| Released | 2011 |
| Latest release | Candidate Recommendation |
| Domain | Cascading Style Sheets |
| Standard | W3C Recommendation |
CSS Selectors Level 3 is a specification developed by the World Wide Web Consortium that defines a rich set of selectors for matching elements in HTML, XHTML and XML documents for styling and scripting. It extends earlier work from the W3C and complements CSS2 concepts while interacting with technologies such as DOM, XPath, and SVG. Major browser vendors including Mozilla Foundation, Google, Apple Inc., Microsoft, and Opera Software implemented many features, influencing web design practices used by projects like WordPress, Bootstrap, and Drupal.
Overview
CSS Selectors Level 3 formalizes selectors used by Cascading Style Sheets to target document nodes, enabling authors to style content produced by systems such as Apache HTTP Server, Nginx, and Node.js applications. It builds on the community and standards work of groups like the WHATWG, the W3C CSS Working Group, and implementations in engines such as Gecko, Blink, WebKit, and Trident. The specification defines syntax, matching rules, and interactions with the cascade defined in CSS and with scripting interfaces exposed by the Document Object Model.
Selector Types
Selectors in Level 3 include element type selectors (tag names used in HTML5 and XHTML), universal selectors used across documents like those served by Apache Tomcat, class selectors widely used in Bootstrap themes, and ID selectors employed in projects such as jQuery-powered sites. It also formalizes group selectors used by frameworks like Foundation and by content management systems like Joomla! and Magento. The specification references legacy and modern document vocabularies including MathML, SVG, and custom XML languages used by enterprises like IBM and Oracle Corporation.
Combinators and Grouping
Combinators specify relationships between selectors and are crucial to stylesheets produced by teams at organizations such as Facebook, Twitter, and GitHub. The descendant combinator, child combinator, adjacent sibling combinator, and general sibling combinator let authors express relationships implemented in front-end toolchains like Grunt and Gulp. Grouping selectors with commas is common in component libraries like Material Design implementations and responsive toolkits employed by companies like Shopify and Etsy.
Pseudo-classes and Pseudo-elements
Level 3 standardizes a set of pseudo-classes—such as :hover, :focus, :active, :first-child, :last-child, :nth-child()—used extensively in user interfaces built by teams at Microsoft Corporation, Apple Inc., and Google LLC. Pseudo-elements like ::before and ::after enable content injection patterns used in web applications from organizations like Netflix, Airbnb, and LinkedIn. Interaction of these selectors with accessibility frameworks and standards such as WAI-ARIA and guidelines from the World Wide Web Consortium's Web Content Accessibility Guidelines is important for sites managed by institutions like The New York Times and BBC.
Attribute Selectors
Attribute selectors introduced and clarified in Level 3 (for example [attr], [attr=value], [attr~=value], [attr|=value], [attr^=value], [attr$=value], [attr*=value]) are widely used in modern web stacks including React, Angular, and Vue.js. They allow targeting elements by attributes such as data-attributes used across projects by Mozilla Foundation and Wikipedia editors, and localization attributes used by organizations like UNESCO and European Commission web properties.
Specificity and Cascade
The specification codifies the specificity rules that determine which rules win when multiple selectors apply, a concept applied in large codebases at companies like Google LLC, Facebook, Amazon and by open-source projects like Drupal themes and WordPress plugins. Specificity is combined with the cascade, origin and importance rules (including user-agent, user, and author styles), and interactions with inline styles manipulated via JavaScript in applications such as Gmail and Outlook.com.
Browser Support and Implementation Notes
Implementations vary across engines from Mozilla Foundation's Firefox to Google's Chrome, Apple's Safari, Microsoft's Edge, and historic Internet Explorer. Polyfills and transpilers created by communities around Modernizr, Babel, and PostCSS help smooth differences for developers at companies like Spotify and Uber Technologies, Inc.. The specification influenced later work such as Selectors Level 4 and integration with web platform features championed at conferences like WWDC, Google I/O, and Mozilla Summit.