@media only screen and ( max-width: 40em ) /* 640 */
{
    #nav
    {
        position: relative;
    }
        #nav > a
        {
        }
        #nav:not( :target ) > a:first-of-type,
        #nav:target > a:last-of-type
        {
            display: block;
        }
 
    /* first level */
 
    #nav > ul
    {
        height: auto;
        display: none;
        position: absolute;
        left: 0;
        right: 0;
    }
        #nav:target > ul
        {
            display: block;
        }
        #nav > ul > li
        {
            width: 100%;
            float: none;
        }
 
    /* second level */
 
    #nav li ul
    {
        position: static;
    }
}