A CSS pseudo-class is a keyword added to a selector that specifies a special state of the selected element(s). For example, the pseudo-class :hover can be used to select a button when a user's pointer hovers over the button and this selected button can then be styled.| MDN Web Docs
The :is() CSS pseudo-class function takes a selector list as its argument, and selects any element that can be selected by one of the selectors in that list. This is useful for writing large selectors in a more compact form.| MDN Web Docs
The CSS selector represents a particular pattern of element or elements in a tree structure. The term "selector" can refer to a simple selector, a compound selector, or a complex selector. When included in the :has() pseudo-class as a parameter, these selectors are referred to as relative selectors, representing elements relative to one or more anchor elements.| MDN Web Docs
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Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media.| MDN Web Docs
1. Introduction| drafts.csswg.org
1. Introduction| drafts.csswg.org