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