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;
}