The :focus selector is used to select the element that has focus.

The :focus selector is allowed on elements that accept keyboard events or other user inputs.

/* example */

:focus{
    outline: 3px solid black;
    outline-offset:3px;
}

/* Cible n'importe quel élément <input> */
/* uniquement lorsqu'il a le focus */
input:focus {
  color: red;
}