make an drodown menu
/*The css file*/
nav {
background-color: black;
max-width: 300px;
margin: 10px;
}
nav li ul {
visibility: hidden;
height: 0;
}
nav > ul > li:hover ul {
visibility: visible;
height: 100px;
width: 60px;
transition-delay: 0.5s;
}