CSS Selectors Level 4: Specificity, Performance, and Scoping
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The provided text offers a comprehensive technical overview of CSS Selectors Level 4, detailing its fundamental role in binding style declarations to elements and its broad utility across CSS and JavaScript DOM querying. It focuses heavily on new and enhanced selectors, such as the relational selector :has() (often referred to as a "parent selector"), the matches-any pseudo-classes :is() and :where(), and expanded negation capabilities with :not(), which now accept selector lists. A significant portion of the text explains the intricate rules for specificity calculation in Level 4, particularly how the new functional pseudo-classes adopt the specificity of their most specific argument, or, in the case of :where(), contribute zero specificity. Furthermore, the source examines browser implementation and performance strategies, including the right-to-left matching algorithm and sophisticated style invalidation techniques necessary to efficiently handle state changes from dynamic pseudo-classes like :hover and :active or manage style scoping within Shadow DOM boundaries
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