In CSS, a pseudo-element is a keyword added to a selector that allows you to style a specific part of an element. Pseudo-elements are used to style elements in ways that can’t be achieved using normal selectors. They are preceded by two colons (::).
Pseudo-elements allow you to style parts of an element that are not part of the actual document tree. For example, you can use the ::before and ::after pseudo-elements to insert content before and after an element’s content, respectively, without adding extra markup to the HTML.
Example:
/* Adding a double border to the first line of a paragraph */
p::first-line {
border-bottom: 2px double black;
}