Selector is pseudo-class

About

:is() ¹⁾ is a matches-any logical pseudo-class that takes a selector list as its argument.

Example

matches any element that is being hovered/focused

in all namspace

matches any element that is being hovered or focused, regardless of its namespace.

*|*:is(:hover, :focus)

in the default namespace

represents only hovered or focused elements that are in the default namespace, because it uses an explicit universal selector within the :is() notation

*|*:is(*:hover, *:focus)

¹⁾

is documentation