nav ul li {
    transition: background-color 0.2s;
    -webkit-transition: background-color 0.2s;
    -moz-transition: background-color 0.2s;
    -o-transition: background-color 0.2s;
    -ms-transition: background-color 0.2s;
}

    nav ul li:hover {
        transition: background-color 0.2s; 
        -webkit-transition: background-color 0.2s;
        -moz-transition: background-color 0.2s;
        -o-transition: background-color 0.2s;
        -ms-transition: background-color 0.2s;
    }