CSS selection Pseudo element
The ::selection pseudo-element is one of the so-called “highlight pseudo-elements” defined by the CSS Pseudo-Elements Module Level 4 specification. Formerly part of the Selectors Level 3 specification, it’s the only highlight pseudo-element that’s currently supported by browsers.
With ::selection, we can apply CSS styles to content that users have highlighted with their mouse. By default, the background and text color of highlighted content is determined by the user’s system settings. Developers, however, can change what that highlight looks like.