CSS ::selection Selector
Make the selected text red on a yellow background:
Definition and Usage
The ::selection selector matches the portion of an element that is selected by a user.
Only a few CSS properties can be applied to the ::selection selector: color, background, cursor, and outline.
Note: The ::selection pseudo-element was drafted for CSS Selectors Level 3, but removed before the Recommendation status. So, at the moment, the ::selection pseudo-element is not in any specification. (However, it may be re-added to future CSS specifications.)