CSS pseudo-elements are keywords added to a selector that allow you to style a specific part of the selected element(s) . They are used to target specific parts of an element, such as the first letter or line, or to insert content before or after the content of an element. Pseudo-elements are denoted by the double colon notation (::) . Some commonly used pseudo-elements include:

• ::first-line: Applies styles to the first line of a block-level element.
• ::first-letter: Applies styles to the first letter of the first line of a block-level element, but only when not preceded by other content (such as images or inline tables) .
• ::before: Inserts content before the content of an element.
• ::after: Inserts content after the content of an element.

Its important to note that pseudo-elements cannot be used to select an element based on its state, but they can be used to select and style a part of an element that already has a state applied to it. Additionally, the double colon notation was introduced with CSS3 to differentiate between pseudo-elements and pseudo-classes, which use a single colon notation (:). However, for backward compatibility, the single colon syntax is still acceptable for CSS2 and CSS1 pseudo-elements.