CSS selectors match elements based on their type, attributes, and relationship to other elements. They define the specific elements to which a block of styles will be applied. This feature represents the oldest selectors of CSS.

The :empty CSS pseudo-class matches elements without child elements.

The :lang() CSS functional pseudo-class matches elements based on their content language.

The :root pseudo-class matches the root element of the document, usually the <html> element.

The :target CSS pseudo-class matches the element with an ID matching the URL fragment.

The :checked, :disabled, and :enabled CSS pseudo-classes match form elements based on their state.

The :active, :focus, and :hover CSS pseudo-classes match elements based on how users are interacting with them.

The :nth-of-type() and :nth-last-of-type() CSS functional pseudo-classes match elements based on their position among siblings of the same type. The :first-of-type, :last-of-type, and :only-of-type pseudo-classes match the first, last, and only elements of its type.

The form validity CSS pseudo-classes match <form> elements based on the constraints of a form field, such as validity (:valid, :invalid, :in-range, :out-of-range) and necessity (:optional or :required).

The @namespace CSS rule sets a default namespace or namespace prefix. Namespace prefixes allow CSS selectors to distinguish elements with the same name but different document types, such as the HTML <a> element and the SVG <a> element.

The ::first-letter CSS pseudo-element selects the first letter in an element for styling.

The ::first-line CSS pseudo-element selects the first line of text in an element for styling.

The :nth-child() and :nth-last-child() CSS functional pseudo-classes match elements based on their index within a list of elements. The :first-child and :last-child pseudo-classes match the first and last element in a list, and the :only-child pseudo-class matches an element with no siblings.